If the equation of the hyperbola with foci (4, 2) and (8, 2) is 3x2 - y2 - x + y + = 0, then + + is equal to .
Answer
141
Solution
Centre = (6, 2), distance between foci 2ae = 4 ⇒ ae = 2
Transverse axis along y = 2:
(x - 6)2a2 - (y - 2)2b2 = 1, with a2 + b2 = (ae)2 = 4
⇒ b2(x - 6)2 - a2(y - 2)2 = a2b2
Comparing the coefficients of x2 and y2 with 3x2 - y2: b2 = 3a2
⇒ 4a2 = 4 ⇒ a2 = 1, b2 = 3
⇒ 3(x - 6)2 - (y - 2)2 = 3
⇒ 3x2 - 36x + 108 - y2 + 4y - 4 - 3 = 0
⇒ 3x2 - y2 - 36x + 4y + 101 = 0
⇒ = 36, = 4, = 101
∴ + + = 141
Answer: 141