Let y = y(x) be the solution of the differential equation (x2 + 4)2dy + (2x3y + 8xy - 2)dx = 0. If y(0) = 0, then y(2) is equal to
- 2
- 8
- 16
- 32
Answer
(4) 32
Solution
2x3y + 8xy = 2xy(x2 + 4) ⇒ dydx + 2xx2 + 4y = 2(x2 + 4)2 (linear)
2xx2 + 4 dx = e(x2 + 4) ⇒ I.F. = e e(x2 + 4) = x2 + 4
y(x2 + 4) = 2x2 + 4dx = -1 x2 + c
y(0) = 0 c = 0
x = 2: 8y = -11 = 4 y(2) = 32
Answer: (4)