let
cosa = 3/5
=> sina =4/5
3sinx－4cosx＝5sin(x＋φ)
=sin(x＋φ)
(3/5)sinx-(4/5)cosx = sin(x＋φ)
sin(x-a) = sin(x＋φ)
φ = -arccos3/5
tanφ = -4/3