And cos (π-a) =-cosasin (3π-a) = sin (2π+π-a) = sin (π-a).

sin(-π-a)=sin-(π+a)= -sin(π+a)

sin(9π/2+a)=sin(4π+？ π+a)=sin(？ π+a)=cosa

As long as the inductive formula is memorized, it can be used quickly.

cos(π—a)sin(3π—a)sin(-π—a)sin(9π/2+a)

=-cosa sin(π-a)-sin(π+a)sin 4π+(π/2+a)

(Simplified) =-COSA Sina-SIN (-a) COSA

= -cos？ A sin? a