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**Solve by direct substitution taht the function [tex]\phi[/tex](t) = Asin(wt) + Bcos(wt) is a solution of the second order differential equation [tex]\ddot{\phi}[/tex] = -w[tex]^{2}[/tex][tex]\phi[/tex]. ( Since this solution involves two arbitrary constants - the coeffecients of the sine and consine functions - it is in fact the general solution).**

I'm not sure what is meant by direct substitution..

Can someone help me get started?

Thanks alot!