Question

(1) cos80度乘cos20度+sin80度乘sin20度 (2) cos20度乘cos25度-sin25度乘sin

Answer 1

(1)cos80°×cos20°+sin80°×sin20°=cos(80°-20°)=cos60°=1/2

(2)cos20°×cos25°-sin25°×sin20°=cos(25°+20°)=cos45°=√2/2

(3)cos22.5°×cos22.5°-sin22.5°×sin22.5°=cos(22.5°+22.5°)=cos45°=√2/2

(4)cos²15°-sin²15°=cos(2×15°)=cos30°=√3/2