Solving separable differential equation Seems straight-forward but I've been unable to get it right. Here are my steps: y'(x) = \(-2y(x) + 28), quad y(-4)=-4

Sophie Marks

Sophie Marks

Answered question

2022-11-04

Seems straight-forward but I've been unable to get it right. Here are my steps:
y ( x ) = 2 y ( x ) + 28 , y ( 4 ) = 4
1 28 2 y d y = d x
28 2 y = x + c
( 28 2 y ) = ( x + c ) 2
y = 1 / 2 x 2 c x c 2 / 2 + 14
c = 2 , 10
y = 1 / 2 x 2 10 x 36
I've checked it over many countless times but for the life of me I can't figure out why it won't work. I've tried plugging the result back into the original equation and it seems to me like it checks out if you take the negative of the square root..

Answer & Explanation

Marshall Flowers

Marshall Flowers

Beginner2022-11-05Added 20 answers

y ( x ) = 28 ( x 2 ) 2 2 = x 2 2 + 2 x + 12.
However, this formula describes a parabola y ( x ) which hits its highest point y = 14 at x = 2, and this is a decreasing function for x > 2, which disagrees with the ODE. (Since the square root in the right-hand side of the ODE can't be negative, the solution y ( x ) can't be decreasing.) Moreover, the right-hand side of the ODE is undefined for y > 14, so the solution can't continue to increase beyond 14. The conclusion is that the only possible continuation is y ( x ) = 14 for all x > 2. Hence, the solution is:
y ( x ) = 28 ( x 2 ) 2 2 = x 2 2 + 2 x + 12 , 4 x 2 ,
y ( x ) = 14 , x > 2.
kituoti126

kituoti126

Beginner2022-11-06Added 8 answers

Your second line should be
1 28 2 y d y = d x .

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