Let y=y(x) be the solution of the differential equation ( x)1/2 dy= ( 3x-( x)3/2y )dx, 0<x< 2, y ( 4 )= 6 25. If y ( 3 )= 45 , then 4 equals .
Answer
48
Solution
Dividing by ( x)1/2dx gives the linear equation dydx + ( x) y = 3x x, with I.F. = e x dx = x, so
y x = 4x x dx
Put x = t, 2x dx = dt: 1 + t2 t dt = 2t1/2 + 25t5/2 + C, hence
y x = 2( x)1/2 + 25( x)5/2 + C
At x = 4: 6 25 2 = 2 + 25 + C, so C = 0.
At x = 3 ( x = 3, x = 2): 2y = 2 31/4 + 25 35/4 = 165 31/4
y = 85 31/4 = 45 = 2 31/4
4 = 16 3 = 48
Answer: 48