6.求证；
1.cos的4次方a/2-sin的4次方a/2=cosa
2.sinacos3次方a-sin3次方acosa=1/4sin4a
3.1+sin2a/sina+cosa=sina+cosa
4.tan（派/4+a）-tan（派/4-a）=2tan2a
5.cota-tana=2cot2a
6.1+sin2a-cos2a/1+sin2a+cos2a=tan2a
7.sin4次方x=1/8（3-4cos2x+cos4x）

1
=[cos²(a/2)+sin²(a/2)][cos²(a/2)-sin²(a/2)]
=cos²(a/2)-sin²(a/2)
=cosa

2,
=sinacosa[cos²a-sin²a)
=(1/2)sin2acos2a
=(1/4)sin4a

3,
=(sin²a+cos²a+2sinacosa)/(sina+cosa)
=(sina+cosa)²/(sina+cosa)
=sina=cosa

4
=(1+tana)/(1-tana) +(1-tana)/(1+tana)
=4tana/(1-tan²a)
=2tana

5
=(1/tana) -tana
=(1-tan²a)/tana
=(1/2)cot2a

6
=[(sina+cosa)² -(cosa+sina)(cosa-sina)]/[(sina+cosa)² +(cosa+sina)(cosa-sina)]
=[(sina+cosa)2sina]/[sina+cosa)2cosa]
=2tana

7
1/8（3-4cos2x+cos4x)
=(1/8)(3 -4cos2x +cos²2x-sin²2x)
=(1/8)[3-4cos²x+4sin²x+(1-2sin²x)²-(2sinxcosx)²]
=(1/8)[4(1-cos²x)-1+4sin²x+1+4(sin²x)²-4sin²x
-4(sinxcosx)²]
=(1/8)[4sin²x +4(sin²x)²-4sin²x(1-sin²x)]
=(1/8)*8(sin²x)²
=左边