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Complex Numbers question

2012 · Shift 0 · Q43
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Complex Numbers question

2012 · Shift 0 · Q43

JEE MainMathematicsComplex NumbersMCQ+4 / −1
If ze1z e 1ze1 and  z2z−1 \,{{{z^2}} \over {z - 1}}\,z−1z2​ is real, then the point represented by the complex number z lies :
  1. A
    either on the real axis or a circle passing through the origin.
  2. B
    on a circle with centre at the origin
  3. C
    either on real axis or on a circle not passing through the origin.
  4. D
    on the imaginary axis.
View written solutionFree

Correct answer: A

  1. Let z=x+iy,z=x+iy,z=x+iy, where x,y∈Rx,y\in\mathbb{R}x,y∈R and z≠1z\ne 1z=1.

  2. Given that z2z−1\frac{z^2}{z-1}z−1z2​ is real. A complex number is real iff its imaginary part is zero.

  3. Compute: z2=(x+iy)2=x2−y2+2ixy,z^2=(x+iy)^2=x^2-y^2+2ixy,z2=(x+iy)2=x2−y2+2ixy, and z−1=(x−1)+iy.z-1=(x-1)+iy.z−1=(x−1)+iy.

    So, z2z−1=(x2−y2+2ixy)(x−1)+iy.\frac{z^2}{z-1}=\frac{(x^2-y^2+2ixy)}{(x-1)+iy}.z−1z2​=(x−1)+iy(x2−y2+2ixy)​.

  4. Rationalize the denominator: z2z−1=(x2−y2+2ixy)((x−1)−iy)(x−1)2+y2.\frac{z^2}{z-1}=\frac{(x^2-y^2+2ixy)\big((x-1)-iy\big)}{(x-1)^2+y^2}.z−1z2​=(x−1)2+y2(x2−y2+2ixy)((x−1)−iy)​.

    We only need the imaginary part of the numerator.

  5. Let A=x2−y2,B=2xy.A=x^2-y^2,\qquad B=2xy.A=x2−y2,B=2xy. Then numerator is (A+iB)((x−1)−iy).(A+iB)\big((x-1)-iy\big).(A+iB)((x−1)−iy).

    Its imaginary part is B(x−1)−Ay.B(x-1)-Ay.B(x−1)−Ay.

    Therefore, 2xy(x−1)−(x2−y2)y=0.2xy(x-1)-(x^2-y^2)y=0.2xy(x−1)−(x2−y2)y=0.

  6. Simplify: 2x2y−2xy−x2y+y3=02x^2y-2xy-x^2y+y^3=02x2y−2xy−x2y+y3=0 x2y−2xy+y3=0x^2y-2xy+y^3=0x2y−2xy+y3=0 y(x2−2x+y2)=0.y(x^2-2x+y^2)=0.y(x2−2x+y2)=0.

    Hence either y=0y=0y=0 or x2−2x+y2=0.x^2-2x+y^2=0.x2−2x+y2=0.

  7. Interpret geometrically:

    • y=0y=0y=0 represents the real axis.
    • x2−2x+y2=0x^2-2x+y^2=0x2−2x+y2=0 x2−2x+1+y2=1x^2-2x+1+y^2=1x2−2x+1+y2=1 (x−1)2+y2=1,(x-1)^2+y^2=1,(x−1)2+y2=1, which is a circle with centre (1,0)(1,0)(1,0) and radius 111.
  8. Check whether this circle passes through the origin: At (0,0)(0,0)(0,0), (0−1)2+02=1, (0-1)^2+0^2=1,(0−1)2+02=1, so the origin lies on the circle.

  9. Therefore, the point represented by zzz lies either on the real axis or on a circle passing through the origin.

  10. Hence the correct option is: A\boxed{\text{A}}A​

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