求值:1-sin^6a-cos^6a/1-sin^4a-cos^4a

最后结果是1.5吧?
1=(sin^2a+cos^2a)^2=sin^4+2sin^2a×cos^2a+cos^4a可以明白?
1-sin^6a-cos^6a sin^4a+2sin^2a×cos^2a+cos^4a-(sin^6a+cos^6a)
--------------- = --------------------------------------------
1-sin^4a-cos^4a sin^4a+2sin^2a×cos^2a+cos^4a-(sin^4a+cos^4a)
sin^4+2sin^2a×cos^2a+cos^4a-(sin^6a+cos^6a)
= --------------------------------------------①
2sin^2a×cos^2a
根据立方和公式:
sin^6a+cos^6a=(sin^2a)^3+(cos^2a)^3
=(sin^2a+cos^2a)(sin^4a-sin^2a×cos^2a+cos^4a)
=sin^4a-sin^2a×cos^2a+cos^4a②
把①中sin^6a+cos^6a换成②中的结果可以得:
sin^4+2sin^2a×cos^2a+cos^4a-(sin^4a-sin^2a×cos^2a+cos^4a)
原式=-------------------------------------------------------------
2sin^2a×cos^2a

3sin^2a×cos^2a
= -----------------=1.5
2sin^2a×cos^2a
不知道结果对不对.但是思路是正确的.
输入的时候排版很好.可是一发布上去就乱了..
你可以看懂不?如果看不懂的话.你给我发消息..
就点儿乱.如果认真看一下还是可以看懂的.
--------是分数线...真是抱歉~