If the locus of z C, such that Re ( z-12z+i ) + Re ( z-12 z-i ) = 2, is a circle of radius r and center (a, b), then 15abr2 is equal to :
- 16
- 24
- 12
- 18
Answer
(4) 18
Solution
z-12 z-i = ( z-12z+i ), and a number and its conjugate have the same real part.
2 Re ( z-12z+i ) = 2 Re ( z-12z+i ) = 1
Put z = x + iy:
(x-1) + iy2x + i(2y+1), real part = 2x(x-1) + y(2y+1)4x2 + (2y+1)2
2x2 - 2x + 2y2 + y = 4x2 + 4y2 + 4y + 1
2x2 + 2y2 + 2x + 3y + 1 = 0
x2 + y2 + x + 32y + 12 = 0
Centre (a, b) = (- 12, - 34 )
r2 = 14 + 916 - 12 = 516
15abr2 = 15 38 516 = 18
Answer: (4)