cos(-40')cos20'-sin(-40')sin(-20')
问题描述：
cos(-40')cos20'-sin(-40')sin(-20')
答
因为y=sinx为奇函数,y=cosx为偶函数,所以原式即：cos40'cos20'-sin40'sin20'.根据两角余弦公式可知：此式=cos(40'+20')=cos60'=1/2.
cos(-40')cos20'-sin(-40')sin(-20')
因为y=sinx为奇函数,y=cosx为偶函数,所以原式即：cos40'cos20'-sin40'sin20'.根据两角余弦公式可知：此式=cos(40'+20')=cos60'=1/2.