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

问题描述:

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

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