Hi there, I don't nee help with solving a question, so much as understanding a step in the provided worked solution. It's using variation of parameters to solve the ode y''+ y = g(t). I've attached the steps in the picture file, and the bit after the word 'Now' what are they doing to the first matrix in order to get the one following the arrow? It might be really obvious but I don't quite get what they've done.
Aso, since y1 = eit and y2 = e-it, shouldn't the terms in the very first matrix listed (before the ones I referred to earlier) be cos(t)+isin(t) (eg. for the first entry) etc, instead of just cos(t)?
The Attempt at a Solution