In Problems 1 through 6, express the solution of the given initial value problem as a sum of two oscillations as in Eq. (8). Throughout, primes denote derivatives with respect to time t. In Problems 1–4, graph the solution function x(t) in such a way that you can identify and label (as in Fig. 5.6.2) its period.

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x" + 25x = 90 cos 4t; x (0) = 0, x'(0) = 90