数学cos20+cos60+cos100+cos140=?

问题描述:

数学cos20+cos60+cos100+cos140=?
如题
过程

=cos20+0.5-sin10-cos40
=sin70+.05-sin(40-30)-cos40
=sin(40+30)+.05-sin40cos30+sin30cos40-cos40
=sin40cos30+sin30cos40+.05-sin40cos30+sin30cos40-cos40
=0.5+0.5cos40+0.5cos40-cos40
=0.5