sin(30'+a)=1/3,求cos(120'-2a)的值_数学解答

sin(30'+a)=1/3,求cos(120'-2a)的值

人气：218 ℃　时间：2020-01-30 09:22:43

解答

cos(2a+60)
=cos[2(a+30)]
=1-2sin²(a+30)
=7/9
cos(120-2a)
=cos[180-(2a+60)]
=-cos(2a+60)
=-7/9