If z1, z2 are two distinct complex numbers such that | z1 - 2z2 12 - z1 z2 | = 2, then
- either z1 lies on a circle of radius 12 or z2 lies on a circle of radius 1.
- z1 lies on a circle of radius 12 and z2 lies on a circle of radius 1.
- either z1 lies on a circle of radius 1 or z2 lies on a circle of radius 12.
- both z1 and z2 lie on the same circle.
Answer
(3) either z1 lies on a circle of radius 1 or z2 lies on a circle of radius 12.
Solution
|z1 - 2z2| = 2 | 12 - z1 z2 | = |1 - 2z1 z2|
Squaring with |w|2 = w w:
(z1 - 2z2)( z1 - 2 z2) = (1 - 2z1 z2)(1 - 2 z1z2)
∵ -2z1 z2 - 2 z1z2 occurs on both sides and cancels
|z1|2 + 4|z2|2 = 1 + 4|z1|2|z2|2
(|z1|2 - 1)(1 - 4|z2|2) = 0
|z1| = 1 or |z2| = 12
∴ either z1 lies on a circle of radius 1 or z2 lies on a circle of radius 12
Answer: (3)