Outside the high school program / Complex geometry 24 exercises (including 19 corrected)

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1. Plan configuration E.3811 The complex plane is pro-vided with a direct orthonormal reference frame O ; u ; v ; (graphic unit : 2 cm ) . 1 Show that the points A of affix 1+i · 3 and B of affix 1 i · 3 are on the same circle with center O whose radius we’ll specify. Draw this circle and then construct the points A and B . 2 Let O be the image of the point O by the rotation r 1 of center A and angle . ı 2 and B the image of the point B by the rotation r 2 with center A and angle + ı 2 . Calculate the affixes of the points O and B and con-struct these points. E.3840 The complex plane is referred to a direct orthonormal reference frame O ; u ; v of graphic unit 1 cm . Make a figure to be completed as the questions are asked. 1 Place the points A , B and C with affixes : z A = 11 + 4 · i ; z B = 3 4 · i ; z C = 5 + 4 · i 2 Calculate the modulus and an argument of the quotient z A z B z C z B and deduce the nature of the triangle ABC . 3 Let E be the image of point C by the rotation R of center B and angle ı 4 . Show that the affix of E verifies : z E = 3 + 8 2 4 · i . Place the point E . 4 Let D be the image of the point E by the homothety H of center B and ratio 2 2 . Show that D is the center of the circumscribed circle of the triangle ABC . Place the point D . E.3842 The complex plane is equipped with a direct orthonormal reference frame O ; u ; v of graphic units 2 cm . Consider the points A and B with re-spective affixes : z A = 1 + i · 3 ; z B = 2 · i 1 a Write z A and z B in exponential form. b Place points A and B on a figure that will be com-pleted during the exercise. c Determine the nature of triangle OAB . 2 We denote r the rotation with center O that transforms A into B . For any point M of affix z , we denote M the image of M by r and z the affix of the point M . a Calculate an argument of the quotient z B z A . Interpret this result geometrically. b Deduce the complex rotation notation r . 3 Let Γ be the circle with center A passing through O and Γ the circle with center B passing through O . Let C be the second point of intersection of Γ and Γ (other than O ) . Let z C be its affix. a Justify that the circle Γ is the image of the circle Γ by rotation r . b Calculate the affix z I of the center I of [ AB ] . c Determine the nature of the quadrilateral OACB . d Deduce that I is the midpoint of [ OC ] then show that the affix of C is : z C = 1 + 2 + 3 · i 4 Let D be the affix point z D =2 · i · 3 . a Justify that the point D belongs to the circle Γ . Place D on the figure. b Place of image D by rotating r as defined in question 2 . We note z D the affix of D . Show that : z D = 3 + 3 · i . 5 Show that the vectors DC and DD are collinear. What can we deduce from this? E.3819 The complex plane is referred to a direct orthonormal reference frame O ; u ; v ; i denotes the complex number of modulus 1 and argument ı 2 Let the points A , B and C have affixes : i ; 1 + i ; 1 + i , respectively. Let f be the application which, to any point M of the plane other than A , of affix z , associates the point M of the plane of affix z such that : z = i · z + 2 z i 1 a Determine images from B and C via the f applica-tion. b Show that, for any complex number z other than i , we have the relationship : ( z i)( z i) = 1 c Let D be the point with affix 1+2 · i . Place the points A , B , C and D on a figure (graphic unit 4 cm ) . Deduce from the previous question a construction of the point D image of the point D by the application f . 2 Let R be a strictly positive real number. What is the image by the application f of the circle with center A and radius R ? 2. Plane transformations https://chingmath.fr chapExoCorrec/3811 sacados/3811 Extrait Pondichery Avril 2004 chapExoCorrec/3840 sacados/3840 Extrait Antilles-Guyane Septembre 2009 chapExoCorrec/3842 sacados/3842 chapExoCorrec/3819 sacados/3819 Extrait La Reunion Juin 2004
E.3820 The complex plane is referred to a O ; u ; v orthonormal reference frame. 1 Determine the exponential form of the complex number: 2 4 · 1 + i 2 Consider the transformation f of the plane which, to any point M of affix z , associates the point M of affix z such that : z = 2 4 · 1 + i · z Determine the nature and characteristic elements of the transformation f . E.3821 The complex plane is referenced to O ; u ; v orthonormal direct. Consider the point I of affix i and the point A of affix : z = 3 + 2 · i 1 Show that the point A belongs to the circle Γ with center point I and radius 2 . On a figure (graphic unit 1 cm ) to be completed as the exercise progresses, place the point I , draw the circle Γ , then construct the point A . 2 Consider the rotation r with center point I and angle ı 2 . Show that point B , image of point A by rotation r , has affix : z B = 1 + i · 3 + 1 Justify that the point B belongs to the circle Γ . 3 Calculate the affix of the point C image of the point A by symmetry of center I . 4 What is the nature of the triangle ABC ? Justify. E.3132 Parts A and B are indepen-dent Consider the equation ( E ) : z 3 (4 + i) z 2 + (7 + i) z 4 = 0 where z denotes a complex number. Part A 1 a Show that ( E ) has a real solution, denoted by z 1 . b Determine the two complex numbers a and b such that, for any complex number z , we have : z 3 (4+i) · z 2 +(7+i) · z 4 = z z 1 · z 2 2i · az + b 2 Solve ( E ) . Part B In the plane equipped with a direct orthonormal coordinate system O ; u ; v , consider the three points A , B and C with respective affixes 1 , 2+2i and 1 i . 1 Represent A , B and C . 2 Determine the modulus and an argument of 2+2 i 1 i . De-duce the nature of triangle OBC . 3 What does line ( OA ) represent for triangle OBC ? Jus-tify your answer. 4 Let D be the image of O under the rotation by angle ı 2 with center C . Determine the affix of D . 5 What is the nature of OCDB ? https://chingmath.fr sacados/3820 sacados/3821 Extrait de France Septembre 2010 chapExoCorrec/3132 sacados/3132
E.3120 The complex plane is referred to a direct orthonormal reference frame O ; u ; v . The points introduced in the text (graphic unit : 2 cm ) will be placed on the same figure, which will be completed as we go along 1 a Solve equation : ( E ): z 2 2 3 · z +4=0 b Consider the complex numbers : z 1 = 3 + i ; z 2 = 3 i And we denote by M and N the points of affixes z 1 and z 2 respectively. Determine the modulus and argument of z 1 and z 2 ; place M and N on the figure. c Determine the affixes of the points Q and P respec-tive images of M and N by the vector translation w = 2 · u . Place P and Q on the figure. Show that MNPQ is a square. 2 Let R be the symmetrical of P with respect to O , E the image of P by the rotation of center O and angle ı 2 , S the image of E by the homothety of center O and ratio 3 . Place these points on the figure. Calculate the affixes of R and S . Show that S belongs to segment [ MN ] . 3 We pose : a =2 3 : a Show that : 1+ a 2 =4 a ; 1 a 2 =2 a 3 b Express the affixes Z of PR and Z of PS in terms of a . c Show that : Z = Z ; Z Z = e i π 3 d Deduce from the previous questions the nature of the triangle PRS E.3136 1 The complex plane is referred to a direct orthonormal reference frame O ; u ; v . We pose : a =3 ; b =5 2 · i ; c =5+2 · i We denote by A , B and C the points with affixes a , b and c respectively. Let M be a point of affix z in the plane, distinct from points A and B . a Show that ABC is an isosceles right-angled triangle. b Give a geometric interpretation of the argument of the complex number z 3 z 5+2 · i . c Then determine the set of points M of affix z such that z 3 z 5+2 · i is a strictly negative real number. 2 Let Γ be the circumscribed circle of the triangle ABC and Ω the point with affix 2 i . a Give the complex writing of the rotation r of center Ω and angle ı 2 . b Determine the image Γ of Γ by the rotation r . Deter-mine a parametric equation of Γ . E.3182 1 In the complex plane referred to an orthonormal refer-ence frame O ; u ; v , consider the points : A of affix a where a R ; B affix b +i where b R ; C image of B by the rotation of center A and angle ı 3 a Determine a relationship between a and b so that the point C belongs to the axis O ; v . b Then express the affix of the point C as a function of a . 2 In this question, we pose a = 3 and b =0 . Consider the points C of affix c = i and D of affix d =2+ 3 2i 3 a What is the nature of the triangle ABC ? b Calculate the quotient d a c a ; what can be deduced for the triangle ACD ? c Determine the affix of the point E image of D in the rotation of center A and angle ı 3 . d Determine the affix of the point F image of D in the translation of vector AC . e Determine the nature of the triangle BEF . E.3202 The complex plane is referenced to a direct orthonormal O;; u ; v . The graphic unit is 2 cm . Designate by i the complex number of modulus 1 and argu-ment + ı 2 . We’ll create a figure and complete it as we go along. 1 Solve in the set C of complex numbers the equation z 4 z =i . Write the solution in algebraic form. 2 Solve in C the equation z 2 2 z +4=0 . Write the solutions in exponential form. 3 Let A , B , A and D be points in the complex plane with affixes : a = 2 ; b = 4 ; a = 2i ; d = 2 + 2i What is the nature of the triangle ODB ? 4 Let E and F be the points with respective affixes : e = 1 i 3 ; f = 1 + i 3 What is the nature of the quadrilateral OEAF ? 5 Let C be the circle of center A and radius 2 . Let C be the circle with center A and radius 2. Let r be the rotation of center O and angle + ı 2 . a Let E be the image of point E under rotation r . Cal-culate the affix e of point E . b Prove that point E is a point on circle C . c Verify that : e d = 3+2 e d . Deduce that points E , E and D are collinear. 6 Let D be the image of point D under rotation r . Prove that triangle EE D is a right triangle. https://chingmath.fr chapExoCorrec/3120 sacados/3120 Antilles-Guyane Septembre 1998 5 points chapExoCorrec/3136 sacados/3136 chapExoCorrec/3182 sacados/3182 Antilles-Guyane Juin 2006 5 points chapExoCorrec/3202 sacados/3202
E.3855 For each of the following six state-ments, indicate whether it is true or false and provide a justi-fication for your answer. An answer without justification will not earn any points. The complex plane is related to an orthonormal direct refer-ence frame O ; u ; v . 1 Let z be a complex number with argument ı 3 . Proposition 1: ˇ z 100 is a real numberı. 2 Let ( E ) be the set of points M with affix z different from 1 in the plane such that : z 1 z = 1 . Proposition 2: ˇthe set ( E ) is a line parallel to the real axisı. 3 Let r be the rotation by angle ı 2 and whose center K has affix 1+i · 3 . Proposition 3: ˇthe image of the point O by the rota-tion r has affix 1 3 +i · 1+ 3 ı 4 Consider the following equation ( E ) : z 2 + 2 · ı 5 · z + 1 = 0 Proposition 4: ˇThe equation ( E ) has two complex solutions with moduli equal to 1 3. Annals on plane configurations E.4074 The complex plane is referenced to the direct orthonormal coordinate system O ; u ; v ; the graphic unit is 1 cm . 1 Solve the following equation in the set of complex num-bers : z 2 + 4 · z + 8 = 0 Give the solutions in algebraic form, then in trigonomet-ric form. 2 Let A and B be the points on the plane with respective affixes : a = 2 2 · i ; b = a Place these points on a graph that will be completed as the exercise progresses. a Determine the affix c of point C , image of point B by rotation about center O and angle ı 2 . b We note D the image of C by rotation about center A and angle ı 2 ; prove that the affix d of point D is : d = 2 6 · i c Place the points C and D on the graph. What is the nature of the quadrilateral ABCD ? 3 ¸ being a non-zero real number, we denote by G α , the barycenter of the system : ( A ; 1) ; ( B ; 1) ; ( C ; ¸ ) a Express the vector CG α en as a function of the vector BA . b Determine the set of points G α when ¸ décrit the set of non-zero reals. Construct this set. c For what value of ¸ will: G α = D ? 4 It is assumed in this question that ¸ =2 . In this question, any trace of research, however incom-plete, or of unsuccessful initiative, will be taken into ac-count in the assessment. Determine and construct the set of points M of the plane such that : MA MB + 2 · MC = 4 · 2 E.3903 The complex plane is provided with a direct orthonormal reference frame O ; u ; v d graphic unit 2 cm . Consider the points A , B and C of affixes : z A = 2 · i ; z B = 3 + i ; z C = 3 + i 1 Écrire z A , z B et z C sous exponential form. 2 Determine the center and radius of the circle Γ passing through the points A , B et C . 3 Make a figure and place the point A , draw the circle Γ then place the points B et C . 4 a Write the quotient z B z A z C z A sous algebraic form and then in exponential form. b Determine the nature of the triangle ABC . 5 We note r the rotation of center A and angle measuring ı 3 radians. a Show that the point O , image of O par r , has affix 3 i . b Demonstrate that the points C and O sont diametri-cally opposed on the circle Γ . c Trace the image Γ of the circle Γ par the rotation r . d Justify that the circles Γ and Γ se intersect at A et B . 6 a Determine the set ( E ) des points M d’affix z tels that : | z | = z + 3 + i | b Show that points A and B belong to ( E ) . https://chingmath.fr chapExoCorrec/3855 sacados/3855 Extrait Liban Juin 2008 chapExoCorrec/4074 sacados/4074 chapExoCorrec/3903 sacados/3903
ABCA’B’C’PQR E.3217 In the oriented plane equipped with a direct orthonormal reference frame, consider ABC a direct triangle on which three equilateral triangles are con-structed externally: BCA , ACB , and ABC . We consider the points P , Q and R to be the respective centers of gravity of the triangles BCA , ACB and ABC , respectively: We note a , b , c , a , b , c , p , q and r the respective affixes of points A , B , C , A , B , C , P , Q and R . 1 a Translate, with the affixes of the points concerned, that C is the image of A by a rotation whose angle and center will be specified. b Show that : a + b + c = a + b + c . 2 Deduce that : p + q + r = a + b + c . 3 Deduce that the triangles ABC , A B C and PQR have the same center of gravity. 4 Show that : 3( q p ) = ( b c ) + ( c a ) + ( a b ) . We will admit that, similarly: 3( r p ) = ( a c ) + ( b a ) + ( c b ) . 5 Justify the following equalities: a c =e i π 3 ( b c ) ; b a =e i π 3 ( c a ) ; c b =e i π 3 ( a b ) 6 Deduce from questions 4 and 5 that the triangle PQR is equilateral. E.3822 The complex plane is equipped with an orthonormal direct reference frame O ; u ; v (unit 1 cm ) . We will draw a figure that we will complete as we go along with the questions. We consider the points A , B , S , and Ω with respective affixes : a = 2 + 4i ; b = 4 + 2i s = 5 + 5i ; w = 2 + 2i Let h be the homothety with center S and ratio 3 . Let C be the image of point A under h and D be the image of point B under h . 1 a Determine the complex form of h . b Show that point C has affix c =4+2 i and that point D has affix d = 2 4 i . 2 Show that points A , B , C , and D lie on the same circle, whose center and radius will be specified. 3 Demonstrate that line ( S Ω) is the perpendicular bisector of segment [ AB ] . 4 Let P be the midpoint of segment [ AC ] . a Determine the affix p of point P . b Show that ! p d b = 1 2 · i . Use this to figure out the mea-sure of angle BD ; P Ω . 5 Let Q be the midpoint of segment [ BD ] . What does point Ω represent for triangle PQS ? 4. Annals on plane transformations E.3175 In the complex plane referred to a direct orthonormal reference frame O ; u ; v (graphic unit 2 cm ) , consider the points A , B and C with affixes : z A = 2 ; z B = 1 + i 3 ; z C = 1 i 3 Part A 1 a Give the exponential form of z B and then of z C . b Place points A , B and C . 2 Determine the nature of the quadrilateral OBAC . 3 Determine and construct the set D of points M of the plane such that : | z | = | z 2 | Part B To any point M of affix z such that z = z A , we associate the point M of affix z defined by: z = 4 z 2 1 a Solve in C the equation : z = 4 z 2 . b Deduce the points associated with B and C . c Determine and place the point G associated with the center of gravity G of the triangle OAB . 2 a Course question: Prerequisite : the modulus of any complex number z , denoted | z | , verifies | z | 2 = z · z where z is the conjugate of z . Show that : for all complex numbers z 1 and zCOPY 01 2 : z 1 × z 2 = z 1 × z 2 . for any non-zero complex number z : 1 z = 1 | z | https://chingmath.fr chapExoCorrec/3217 sacados/3217 Antilles-Guyane Septembre 2004 5 points ABCA’B’C’PQR sacados/3822 chapExoCorrec/3175 sacados/3175
b Show that for any complex number z distinct from 2 : z 2 = 2 | z | | z 2 | c It is assumed in this question that M is any point of D , where D is the set defined in question 3 from part A . Show that the point M associated with M belongs to a circle Γ whose center and radius will be specified. Trace Γ . E.3156 The plane is referred to a direct orthonormal coordinate system O ; u ; v (graphic unit : 1 cm ) . Part A In the reference frame O ; u ; v , consider the curve H of equation : y 2 x 2 =16 . 1 Show that H is the union of two curves C and C where C is the representative curve of the function f defined on R by f ( x )= x 2 +16 and where C is the image of C by a simple transformation to be specified. 2 a Study the function f (limits at the bounds of the defining set and direction of variation) . b Show that the straight line of equation y = x is an asymptote of C . c Draw H in the reference frame O ; u ; v . We name A and B the points on the curve with abscis-sas 3 and 3 respectively. Consider the domain D of the plane made up of the points M ( x ; y ) verifying: 3 x 3 ; x 2 + 16 y 5 Hatch the domain D and express the area of D using an integral that we won’t attempt to calculate. Part B We call r the rotation of center O and angle ı 4 . 1 a Give the complex writing of r . b Denote by x and y the coordinates of the point M , image of the point M ( x ; y ) in the plane. Check that : x = 1 2 · ( x + y ) y = 1 2 · ( x + y ) Determine the coordinates of points A and B , respec-tive images of A and B by rotation r . Place the points A and B in the reference frame O ; u ; v . 2 Let H be the hyperbola of equation : x · y = 8 . a Trace H in the reference frame O ; u ; v . b Show that H is the image of H by rotation r . 3 Let D be the image of D by rotation r . We admit that D is the set of points M ( x ; y ) of the plane verifying: 2 x 4 · 2 ; 8 x y 5 · 2 x . a Chop D . b Calculate the area of D , expressed in cm 2 . Deduce an approximate value to the nearest 10 3 of the area of D . 5. Autes annales E.3901 The plane is equipped with a di-rect orthonormal reference frame O ; u ; v . Let A , B , and P be the respective affix points : a = 5 + 5 · i ; b = 5 5 · i ; p = 10 Consider a point M , distinct from O , with affix z . We denote U the point with affix u , image of the point M by the rotation R A with center A and angle of measurement ı 2 . We denote T the affix point t , image of point M by rotation R B with center B and angle of measurement ı 2 . Let D be the image of point M by the symmetry of center O . 1 Demonstrate that the affix of point U is u =i(10 z ) ; ex-press in terms of z the affix of point T then justify that quadrilateral MUDT is a parallelogram with center O . 2 Determine the set Γ of points M with affix z such that : z · z 5 · z 5 · z = 0 Justify that the quadrilateral OAPB is inscribed in Γ . 3 We assume that point M is distinct from O , A and P . Points O , M , and U are therefore distinct from each other in pairs. a Prove that points O , M and U are collinear if, and only if : u z = u z b Prove that points O , M and U are collinear if, and only if, M belongs to Γ . 4 Determine the set of points M on the plane such that OMU is an isosceles triangle at O . What is the nature of the quadrilateral MUDT in this case? 5 Determine the set of complex numbers z such that u z is purely imaginary. Deduce the nature of the quadrilateral MUDT in the case where M is a point on the line ( OP ) devoid of O and P . Finally, prove that there is a unique position of the point M such that MUDT is a square. 6. Unclassified financial years https://chingmath.fr chapExoCorrec/3156 sacados/3156 chapExoCorrec/3901 sacados/3901
E.4236 The complex plane is referenced to O ; u ; v orthonormal direct. Consider the point I of affix i and the point A of affix z A = 3+2 · i 1 Show that the point A belongs to the circle Γ with center point I and radius 2 . 2 Consider the rotation r with center the point I and angle ı 2 . Show that the point B image of the point A by the rota-tion r has affix : z B = 1 + i · 3 + 1 Justify that the point B belongs to the circle Γ . 3 Calculate the affix of the point C , image of the point A by central symmetry of the point I . 4 What is the nature of the triangle ABC ? Justify. E.4248 The complex plane is referred to a reference frame O ; u ; v orthonormal direct Consider the points A and B with respective affixes : a = i ; b = 1 + i Note: r A the rotation with center A and angle ı 2 ; r B the rotation of center B and angle ı 2 ; r O the rotation of center O and angle ı 2 . Consider : the point C of affix c =3 · i ; the point D image of C by r A ; point G image of D by r B ; point H image of C by r 0 . Let d , g and h be the respective affixes of the points D , G and H . 1 Demonstrate that d = 2+i . 2 Determine g and h . 3 Demonstrate that the quadrilateral CDGH is a rectan-gle. E.3859 The complex plane is referred to a reference frame O ; u ; v orthonormé. Let ( C ) the circle with center O et and radius 1 . Consider the point A of ( C ) d of affix : z A = e i · π 3 1 Determine the affix z B du point B image of A par the rotation of center O et of angle 2 ı 3 . Determine the affix z C of point C image of B par the rotation of center O et of angle 2 ı 3 . 2 a Justify that ( C ) est the circumscribed circle of the triangle ABC . Construct the points A , B and C on the sheet of graph paper. b What is the nature of the triangle ABC ? Justify. 3 So h the homothety of center O and ratio 2 . a Complete the figure by placing the points P , Q et R images respective of the points A , B et C par h . b What is the nature of the triangle PQR ? Justify. 4 In this question, the candidate is invited to write on his copy the steps of his approach even if it does not succeed . a Give the complex writing of h . b Calculer z A + z B + z C . Deduce that A is the middle of segment [ QR ] . c What can be said of the straight line ( QR ) in relation to the circle ( C ) ? https://chingmath.fr sacados/4236 sacados/4248 Extrait Amerique du Nord Mai 2011 chapExoCorrec/3859 sacados/3859
E.3968 Part 1: Organized restitution of knowledge The complex plane is referred to a O ; u ; v orthonormé direct reference frame. Prerequisites: Remember that the complex writing of a direct similitude in the plane is of the form z = ¸ · z + ˛ , where ¸ est is a non-zero complex number and ˛ est is a complex number. Let A , B , C , D quatre points of the plane ; on the one hand, assume that the points A et C sont distinct and, on the other hand, that the points B et D sont distinct. Show that there is a single direct similitude such that : s ( A ) = B ; s ( C ) = D Part 2: The complex plane is referred to the reference frame A ; AB ; AD orthonormal direct such that : AB ; AD = ı 2 [2 · ı ] Consider point C such that ABCD is a square. Let E be the midpoint of segment [ AD ] , we consider the square EDGF such that : ED ; EF = ı 2 [2 · ı ] . 1 a Draw a figure by placing the points A , B , C , D , E , F , G . The figure will be completed during the exercise. b Specify the complex numbers a , b , c , d , e , f , g , respec-tive affixes of points A , B , C , D , E , F and G . c Show that there is a unique direct similarity s of the plane such that : s ( D ) = F ; s ( B ) = D 2 We propose to specify the characteristic elements of di-rect similarity s . a Determine the ratio k and angle of the direct simi-larity s . b Give the complex form of this similarity. c Determine the center Ω of the direct similarity s . E.3128 The plane is referenced to O ; u ; v direct orthonormal (graphic unit 2 cm ) . The figure will be completed as the exercise progresses. Let I be the point with affix 2i . We call f the transformation which, to any point M of affix z associates the point M of affix z such that z =i · z . 1 a Specify the nature of f and its characteristic ele-ments. b Determine the affix of the point A , image by f of the point A of affix 1+ 2+i . c Show that the points A , I and A are aligned. 2 a Show that the set (Γ) of points M of the plane such that M , I and M are aligned, is the circle with center Ω of affix 1+i and radius 2 . b Verify that the point A belongs to (Γ) . c Determine the set ) described by the point M when the point M describes (Γ) . 3 Let B be the point with affix 2+2i and B be the image of B by f . a Demonstrate that the straight lines ( AB ) and ( A B ) are perpendicular. b Let C be the point of intersection of the lines ( AB ) and ( A B ) . Determine the nature of the quadrilateral OACA . https://chingmath.fr chapExoCorrec/3968 sacados/3968 chapExoCorrec/3128 sacados/3128