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Complex Numbers question

2014 · Shift 0 · Q42
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Complex Numbers question

2014 · Shift 0 · Q42

JEE MainMathematicsComplex NumbersMCQ+4 / −1
If z is a complex number such that  ∣z∣≥2 \,\left| z \right| \ge 2\,∣z∣≥2, then the minimum value of   ∣z+12∣\,\,\left| {z + {1 \over 2}} \right|​z+21​​ :
  1. A
    is strictly greater that 52{{5 \over 2}}25​
  2. B
    is strictly greater that 32{{3 \over 2}}23​ but less than 52{{5 \over 2}}25​
  3. C
    is equal to 52{{5 \over 2}}25​
  4. D
    lie in the interval (1, 2)
View written solutionFree

Correct answer: D

  1. We need the minimum value of ∣z+12∣\left|z+\frac12\right|​z+21​​ given that ∣z∣≥2.|z|\ge 2.∣z∣≥2.

  2. Interpret this geometrically in the Argand plane.

  • The condition ∣z∣≥2|z|\ge 2∣z∣≥2 means the point zzz lies on or outside the circle centered at 000 with radius 222.
  • The quantity ∣z+12∣=∣z−(−12)∣\left|z+\frac12\right|=\left|z-\left(-\frac12\right)\right|​z+21​​=​z−(−21​)​ is the distance of zzz from the fixed point −12-\frac12−21​ on the real axis.

So we want the minimum distance from the point −12-\frac12−21​ to the region ∣z∣≥2|z|\ge 2∣z∣≥2.

  1. The point −12-\frac12−21​ lies inside the circle ∣z∣=2|z|=2∣z∣=2, since ∣−12∣=12<2.\left|-\frac12\right|=\frac12<2.​−21​​=21​<2. Hence the nearest point in the region ∣z∣≥2|z|\ge 2∣z∣≥2 will be on the boundary circle ∣z∣=2|z|=2∣z∣=2, along the line joining the origin and −12-\frac12−21​.

  2. Therefore, the minimum distance is 2−12=32.2-\frac12=\frac32.2−21​=23​.

We can also verify by triangle inequality: ∣z+12∣=∣z−(−12)∣≥∣∣z∣−12∣.\left|z+\frac12\right|=\left|z-\left(-\frac12\right)\right|\ge ||z|-\tfrac12|.​z+21​​=​z−(−21​)​≥∣∣z∣−21​∣. Since ∣z∣≥2|z|\ge 2∣z∣≥2, ∣z+12∣≥2−12=32.\left|z+\frac12\right|\ge 2-\frac12=\frac32.​z+21​​≥2−21​=23​. Equality occurs when zzz is on the negative real axis with ∣z∣=2|z|=2∣z∣=2, i.e. at z=−2.z=-2.z=−2. Then ∣−2+12∣=∣−32∣=32.\left|-2+\frac12\right|=\left|-\frac32\right|=\frac32.​−2+21​​=​−23​​=23​. So the minimum value is exactly 32.\boxed{\frac32}.23​​.

  1. Check options:
  • A: strictly greater than 52\frac5225​ — false
  • B: strictly greater than 32\frac3223​ but less than 52\frac5225​ — false, because minimum equals 32\frac3223​, not strictly greater
  • C: equal to 52\frac5225​ — false
  • D: lies in interval (1,2)(1,2)(1,2) — true, since 32∈(1,2)\frac32\in(1,2)23​∈(1,2)

Thus the correct option is D.\boxed{D}.D​.

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