Use the following Composite Argument Properties
color(red)(cos(A-B)=cos A cos B+sinA sinBcos(A−B)=cosAcosB+sinAsinB
color(red)(sin(A+B)=sinAcosB+cos A sin Bsin(A+B)=sinAcosB+cosAsinB
Left Hand Side:
color(green)(cos(pi-theta)+sin(pi/2+ theta)=color(blue)(cos picos theta+sin pi sin theta+sin (pi/2) cos theta+cos (pi/2)sin theta cos(π−θ)+sin(π2+θ)=cosπcosθ+sinπsinθ+sin(π2)cosθ+cos(π2)sinθ
color(blue)(=-1*cos theta+0*sin theta+1*cos theta+0*sin theta=−1⋅cosθ+0⋅sinθ+1⋅cosθ+0⋅sinθ
color(blue)(=-cos theta+cos theta)=−cosθ+cosθ
color(blue)(=0)=0
color(blue)( :.= Right Hand Side