Let the solution curve of the differential equation x dy - y dx = x2 + y2 dx, x > 0, y(1) = 0, be y = y(x). Then y(3) is equal to
- 6
- 4
- 1
- 2
Answer
(2) 4
Solution
The equation dydx = y + x2 + y2x is homogeneous, so put y = vx, which gives dydx = v + x dvdx and ( x > 0)
v + x dvdx = v + 1 + v2
dv 1 + v2 = dxx
(v + 1 + v2 ) = x + c
At x = 1, y = 0 we have v = 0, so c = 0 and
v + 1 + v2 = x
y + x2 + y2 = x2
At x = 3: y + 9 + y2 = 9
9 + y2 = (9 - y)2 = 81 - 18y + y2
18y = 72 y = 4
y(3) = 4
Answer: (2)