诱导公式
cos(-x)=cosx
cos(a+b)=cosacosb-sinasinb
所以cos(-165)
=cos165
=cos(135+30)
=cos135cos30-sin135sin30
=(-√2/2)(√3/2)-(√2/2)(1/2)
=-(√6+√2)/4
cos(-165）解答过程怎样,急需
诱导公式
cos(-x)=cosx
cos(a+b)=cosacosb-sinasinb
所以cos(-165)
=cos165
=cos(135+30)
=cos135cos30-sin135sin30
=(-√2/2)(√3/2)-(√2/2)(1/2)
=-(√6+√2)/4