y'=[ cosx * 3x- sinx*3 ] /9x^2 = [ 3xcosx-3sinx] /9x^2 = [ xcosx-sinx ] / 3x^2
y'= [ -2sin2x *3x - cos2x*3 ] / 9x^2 = [ -6xsin2x-3cos2x ] /9x^2=
=[ -2xsin2x-cos2x]/3x^3
