1、原式
=[(1-tan²a)/(1+tan²a)]/[(1-tana)/(1+tana)]
=(1-tana)²/(1+tan²a)
=(cosa-sina)²
=1-sin(2a)
2、原式
=2*|sin4-cos4|-2cos4
=2(sin4-cos4)-2cos4
=2sin4-4cos4
3、原式
=[sin7°+(sin23°-sin7°)/2]/[cos7°+(cos23°-cos7°)/2]
=(sin7°+sin23°)/(cos7°+cos23°)
=2sin15°cos8°/(2cos15°cos8°)
=tan15°
=sin30°/(1+cos30°)
=2-√3