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

Adecibel is a logarithmic unit that can be used to measure the intensity of a sound. the function y = 10logx gives the intensity of a sound in decibels, where x is the ratio of the intensity of the sound to a reference intensity. the reference intensity is generally set at the threshold of hearing. use the graphing utility to graph the decibel function.a whisper has a sound intensity of 30 decibels. use your graph to you complete the following statement. the sound intensity of a whisper is times the reference intensity.