求值（1）cos80°cos35°+cos10°cos55° （2）cos^2 15°-sin^2 15°

题目

求值（1）cos80°cos35°+cos10°cos55° （2）cos^2 15°-sin^2 15°

答案

1、cos80°cos35°+cos10°cos55°
=cos80°cos35°+sin80°sin35°
=cos(80°-35°)
=cos45°
=√2/2
2、cos²15°-sin²15°
=cos(2x15°)
=cos30°
=√3/2