Question

Help me. Giúp em bài 3 vs. Cần gấp gấo

Answer 1

a.\(cos\left(\dfrac{-3A+B+C}{2}\right)=cos\left(\dfrac{A+B+C}{2}-2A\right)=cos\left(90^o-2A\right)=sin2A\)

b.\(cos\left(A+B-C\right)=cos\left(A+B+C-2C\right)=cos\left(180^o-2C\right)=-cos2C\)