2) cos19°30'cos25°30' - sin 19°30'sin25°30' 3) cos 8п/7*cos п/7+sin 8п/7*sin п/7

Question

2) cos19°30'cos25°30' - sin 19°30'sin25°30' 3) cos 8п/7*cos п/7+sin 8п/7*sin п/7

Answer 1

2) cos a*cos b-sin a*sin b=cos(a+b)
cos(19°30'+25°30')=cos 45°=√2/2
3) cos a*cos b+sin a*sin b=cos(a-b)
cos(8Π/7-Π/7)=cos Π=-1