SOLUTION: Solve the following initial value problems: A)x"+x=0 :x(0)=0,x'(0)=1 b)x"+4x=0; x(0)=1, x'(0)=0

Algebra ->  College  -> Linear Algebra -> SOLUTION: Solve the following initial value problems: A)x"+x=0 :x(0)=0,x'(0)=1 b)x"+4x=0; x(0)=1, x'(0)=0      Log On


   



Question 358398: Solve the following initial value problems:
A)x"+x=0 :x(0)=0,x'(0)=1
b)x"+4x=0; x(0)=1, x'(0)=0

Answer by robertb(5830) About Me  (Show Source):
You can put this solution on YOUR website!
A) We can let x%28t%29+=+AsinBt. This gives dx%2Fdt+=+AB%2A%28cosBt%29, and d%5E2x%2Fdt%5E2+=+-AB%5E2%2A%28sinBt%29. Then
d%5E2x%2Fdt%5E2+%2B+x+=+AsinBt-AB%5E2%2A%28sinBt%29+=+A%2A%281-B%5E2%29%2AsinBt. This implies that A%281-B%5E2%29+=+0, from the given DE. Since we don't want A = 0, then B = 1. (We can drop B = -1 without loss of generality.)
So x%28t%29+=+Asint. From the initial value x'(0) = 1, we get A = 1. Therefore x%28t%29+=+sint.
B) The solution of this IVP follows along the same line as above, but this time we letx%28t%29+=+AcosBt. By using the differential equation we get B=2, and by using the initial condition, we get A = 1. Therefore x%28t%29+=+cos2t.