If y = y(x) satisfies the differential equation 16 ( 9x + x x ) (4 + 9 + x ) y dy = (1 + 2 y) dx, x > 0 and y(256) = 2, y(49) = , then 2 is equal to :
- 2 2 - 1
- 2 - 1
- 2( 2 - 1)
- 3( 2 - 1)
Answer
(1) 2 2 - 1
Solution
9x + x x = x 9 + x
Separating the variables:
y dy1 + 2 y = dx16 x 9 + x (4 + 9 + x )
Put t = 4 + 9 + x
dt = 12 9 + x 12 x dx
dx x 9 + x = 4 dt
y dy1 + 2 y = 4 dt16 t
12 (1 + 2 y) = 14 t + C
(1 + 2 y)2 = K (4 + 9 + x )
x = 256: x = 16, t = 4 + 5 = 9, y = 2
9 = 9K K = 1
x = 49: x = 7, t = 4 + 4 = 8
(1 + 2 )2 = 8 1 + 2 = 2 2 (positive, as at x = 256)
2 = 2 2 - 1
Answer: (1)