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
Linear equation with P = 2 2 x
Integrating factor = e 2 2 x dx = e2 x
y e2 x = (2 + 3 x) 2 x e2 x dx
Put 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
y(0) = 54 14 + C = 54 C = 1
y ( 4 ) = 14 + 32 + e-2 = 74 + e-2
12 (y ( 4 ) - e-2 ) = 12 74 = 21
Answer: 21