Let (a, b) be the point of intersection of the curve x2 = 2y and the straight line y - 2x - 6 = 0 in the second quadrant. Then the integral I = ab 9x21 + 5x dx is equal to :
- 27
- 18
- 24
- 21
Answer
(3) 24
Solution
Put y = 2x + 6 in x2 = 2y:
x2 = 4x + 12 x2 - 4x - 12 = 0
(x - 6)(x + 2) = 0 x = 6 or x = -2
Second quadrant ⇒ x = -2, y = 2(-2) + 6 = 2
(a, b) = (-2, 2)
I = -22 9x21 + 5x dx
Using -aa f(x) dx = -aa f(-x) dx:
I = -22 9x21 + 5-x dx = -22 9x2 5x5x + 1 dx
Adding the two forms:
2I = -22 9x2 dx = 2 9 233 = 48
I = 24
Answer: (3)