sin^4a+ cos^4a
=(sin²a+cos²a)²-2sin²acos²a
=1²-2sin²acos²a
=1-2sin²acos²a
sin^6a+cos^6a
=(sin²a+cos²a)(sin^4a-sin²acos²a+cos^4a)
=1*(1-2sin²acos²a-sin²acos²a)
=1-3sin²acos²a
所以原式=3-6sin²acos²a-2+6sin²acos²a=1
3(sin^4a+ cos^4a)-2(sin^6a+cos^6a)
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