【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