Let y = y(x) be the solution of the differential equation dydx + 2y 2 x = 2 2 x + 3 x 2 x such that y(0) = 54. Then 12 (y ( 4 ) - e-2 ) is equal to .
Answer
21
Solution
This is a linear equation with P = 2 2 x, so the integrating factor is e 2 2 x dx = e2 x and
y e2 x = (2 + 3 x) 2 x e2 x dx
Putting x = t, 2 x dx = dt:
y e2t = (2 + 3t)e2t dt = e2t + 3 ( t2 - 14 )e2t + C
y = 14 + 32 x + C e-2 x
Since y(0) = 54, 14 + C = 54, so C = 1 and
y ( 4 ) = 14 + 32 + e-2 = 74 + e-2
12 (y ( 4 ) - e-2 ) = 12 74 = 21
Answer: 21