# Homework Help: The Principle of Superposition for Homogeneous Equations (DiffEq)

1. Oct 5, 2014

### _N3WTON_

1. The problem statement, all variables and given/known data
Verify that $e^x$ and $e^-x$ and any linear combination $c_1e^x + c_2e^{-x}$ are all solutions of the differential equation:
$y'' - y = 0$
Show that the hyperbolic sine and cosine functions, sinhx and coshx are also solutions

2. Relevant equations
Principle of Superposition for Homogeneous Equations
$y'' + p(x)y' + q(x)y = 0$
$y(x) = c_1y_1(x) = c_2y_2(x)$

3. The attempt at a solution
I am not having any trouble on the first part, here is my solution:
$y_1(x) = e^x$
$y_2(x) = e^{-x}$
$y'' - y = 0$
$y = c_1e^x + c_2e^{-x}$
$y' = c_1e^x - c_2e^{-x}$
$y'' = c_1e^x + c_2e^{-x}$
$(c_1e^x + c_2e^{-x}) - (c_1e^x + c_2e^{-x}) = 0$
$0 = 0$
Now, on the second part of the problem I run into problems, here is what I have so far:
$y_1(x) = sinh(x)$
$y_2(x) = cosh(x)$
$y = c_1cosh(x) + c_2sinh(x)$
$y' = -c_1sinh(x) + c_2cosh(x)$
$y'' = -c_1cosh(x) - c_2sinh(x)$
$(-c_1cosh(x) - c_2sinh(x)) - (c_1cosh(x) + c_2sinh(x)) = 0$
However, the last equation is not true and I am not sure where I went wrong....

Last edited: Oct 5, 2014
2. Oct 5, 2014

### Staff: Mentor

Your derivative for cosh(x) is wrong. d/dx(cosh(x)) = sinh(x).

3. Oct 5, 2014

### LCKurtz

The derivatives of hyperbolic sinh cosh functions don't have a minus sign like the ordinary sines and cosines do.

4. Oct 5, 2014

### _N3WTON_

thank you both...I was so confused for a moment XD