Verified answer

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

1.cos5βcos2β+sin5βsin2β=cos(5β-2β)=cos3β;

2. cos(7π/5+α)cos(2π/5+α)+sin(7π/5+α)sin(2π/5+α)=cos(7π/5+α-2π/5-α)=cos(5π/5)=cosπ=-1;

3. sin73cos13-cos73sin13=sin(73-13)=sin60=