Chapter 4.2, Problem 22P

In each of problem 22 through 25, verify that the functions $y_1$ and $y_2$ are the solutions of the given differential equation. Do they constitute a fundamental set of solutions?

$y\text{'}\text{'}+4y=0$; $y_1\left(t\right)=\cos 2t$, $y_2\left(t\right)=\sin 2t$