Complex Numbers 2 Question 3

3. Let z1 and z2 be two complex numbers satisfying |z1|=9 and |z234i|=4. Then, the minimum value of |z1z2| is

(2019 Main, 12 Jan II)

(a) 1

(b) 2

(c) 2

(d) 0

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Answer:

Correct Answer: 3. (d)

Solution:

  1. Clearly |z1|=9, represents a circle having centre C1(0,0) and radius r1=9.

and |z234i|=4 represents a circle having centre C2(3,4) and radius r2=4.

The minimum value of |z1z2| is equals to minimum distance between circles |z1|=9 and |z234i|=4.

C1C2=(30)2+(40)2=9+16=25=5

and |r1r2|=94|=5C1C2r1r2|

Circles touches each other internally.

Hence, |z1z2|min=0



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