I guess this is maybe more algebra than calculus, but it stems from a calculus problem, so I'll stick it here. The problem is: In the case of the simple harmonic oscillator the solution [to the EOM] may be written at least 3 ways x(t) = Acos(wt) + Bsin(wt) = Ccos(wt + del) = De^(iwt) + Ee^(1wt) Express C and del in terms of A and B. Express D and E in terms of A and B. What I've got: I got the first part, C and del, but I can't figure out how to find D and E. It seems relatively straightforward, I put the A/B eq as the LHS and the D/E eq on the RHS and just applied Euler's Formula to the D term. But I can't figure out how to get the E term in terms of cos and sin. Any help please?