In each of Problems 17 through 22, find the solution of the given initial value problem. Sketch the graph of the solution and describe its behavior for increasing $t$.
17. $y'' + 4y = 0$, $y(0) = 0$, $y'(0) = 1$
18. $y'' + 4y' + 5y = 0$, $y(0) = 1$, $y'(0) = 0$
19. $y'' - 2y' + 5y = 0$, $y(\pi/2) = 0$, $y'(\pi/2) = 2$