Using direct substitution, verify that y(t) is a solution of the given differential equations 17-19. Then using the initial conditions, determine the constants C or c1 and c2.
17. y ′′ + 4y = 0, y(0) = 1, y ′ (0) = 0, y(t) = c1 cos 2t + c2 sin 2t
18. y ′′ − 5y ′ + 4y = 0, y(0) = 1, y ′ (0) = 0, y(t) = c1et + c2e4t
19. y ′′ + 4y ′ + 13y = 0, y(0) = 1, y ′ (0) = 0, y(t) = c1e-2t cos 3t + c2e-3tsin 3t