cos40° cos60° cos80° cos160°怎樣求值？我要解答過程，越詳細(xì)越好！多謝幫忙！

熱心網(wǎng)友

cos40°+ cos60°+ cos80°+ cos160°怎樣求值？解:cos40°+ cos60°+ cos80°+ cos160°=cos40°-cos20°+cos60°+cos80°=-2sin30°sin10°+1/2+sin10°=-sin10°+sin10°+1/2=1/2.

熱心網(wǎng)友

cos40°+cos160°=2cos100°cos60°=cos100°=-cos80°所以cos40°+cos60°+cos80°+cos160°=cos60°+cos80°-cos80°=cos60°=1/2.