see below
Use Formulas: ##sin (A+B)=sinAcosB+cosAsinB##
##cos(A+B)=cosAcosB-sinAsinB##
##A=x+y B=z##
Left Side:##=sin((x+y)+z)=sin (x+y)cosz+cos(x+y)sinz##
##=(sinxcosy+cosxsiny)cosz+(cosxcosy-sinxsiny)sinz##
##=sinxcosycosz+cosxsinycosz+cosxcosysinz-sinxsinysinz##
##=##Right Side