The equation is

$a\mathrm{cos}\left(\omega t\right)+b\mathrm{sin}\left(\omega t\right)=A\mathrm{cos}(\omega t+\varphi )$

I must have forgotten how the right hand side follows from the left hand side.

Some hints would be helpful

$a\mathrm{cos}\left(\omega t\right)+b\mathrm{sin}\left(\omega t\right)=A\mathrm{cos}(\omega t+\varphi )$

I must have forgotten how the right hand side follows from the left hand side.

Some hints would be helpful