cos20+cos60+cos100+cos140求值

问题描述:

cos20+cos60+cos100+cos140
求值

原式=cos20+cos60+cos(120-20)+cos(120+20)
=cos20+cos60+cos120cos20+sin120sin20+cos120cos20-sin120sin20
=cos20+cos60+2cos120cos20
=cos20+cos60-cos20
=cos60
=1/2