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

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

人气：309 ℃　时间：2020-06-04 01:02:02

解答

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