The sum of all possible values of [- , 2 ], for which 1 + i 1 - 2i is purely imaginary, is equal to :
- 4
- 3
- 2
- 5
Answer
(2) 3
Solution
Multiply by the conjugate 1 + 2i :
(1 + i )(1 + 2i )1 + 4 2 = (1 - 2 2 ) + 3i 1 + 4 2
purely imaginary real part = 0 (imaginary part 0)
2 = 12 ( 0 ✓)
= 4, 3 4, 5 4, 7 4 in [- , 2 ]
Sum = 0 + 0 + 5 4 + 7 4 = 3
Answer: (2)