1：COS(-7π/12) = 2：tan165/(1-tan^215) = 第二题题目打错了，应该是这个：tan165/(1-tan^2 15) =

这个＆#是加还是什么啊 47 12是47度12分么？

1.cos(-7π/12) =cos(-π/3-π/4)=cos(-π/3)cos(-π/4)-sin(-π/3)sin(-π/4)=(√2-√6)/4

2.tan165/(1-tan^2 15),其中tan165=tan(180-15°)=-tan15°=√3-2
1-tan^2 15=1-(7-4√3)=4√3-6,那么tan165/(1-tan^2 15)=（√3-2)/(4√3-6)=-√3/6

、cos(-7π/12) =cos7π/12=cos(π-5π/12)=-cos5π/12=-cos(π/4+π/3)=-（根号6-根号2）/4
2、 tan165/(1-tan^215)=-tan15/(1-tan^215)=-(2-根号3）/[1-(2-根号3)^2]=(1-根号3）/8

1、cos(-7π/12) =cos7π/12=cos(π-5π/12)=-cos5π/12=-cos(π/4+π/3)=-（根号6-根号2）/4
2、 tan165/(1-tan^215)=-tan15/(1-tan^215)=-(2-根号3）/[1-(2-根号3)^2]=(1-根号3）/8
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【参考答案】
1、cos(-7π/12)
=cos(7π/12)
=cos(45°+60°)
=cos45°cos60°-sin45°sin60°
=(√2/2)×(1/2)-(√2/2)×(√3/2)
=(√2-√6)/4
2、tan165°/(1- tan²15°)
=tan(180°-15°)/(1-tan²15°)
=-tan15°/(1-tan²15°)
=(-1/2)×[2tan15°/(1-tan²15°)]
=(-1/2)×tan30°
=(-1/2)×(√3/3)
=-√3/6