Let z be the complex number satisfying |z - 5| 3 and having maximum positive principal argument. Then 34 | 5z - 125iz + 16 |2 is equal to:
- 20
- 26
- 12
- 16
Answer
(1) 20
Solution
|z - 5| 3 is the disc with centre (5, 0) and radius 3, and the argument is maximum at the point where the tangent from O touches the circle.
The tangent length is 52 - 32 = 4, with = 35, = 45, so
z = 4( + i ) = 16 + 12i5
Then 5z - 12 = 4 + 12i and 5iz + 16 = i(16 + 12i) + 16 = 4 + 16i, so
| 5z - 125iz + 16 |2 = 16 + 14416 + 256 = 160272 = 1017
34 1017 = 20
Answer: (1)