化简:cos2a-6(sina/2)^2-8(sina/2)^4

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化简:cos2a-6(sina/2)^2-8(sina/2)^4
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化简:cos2a-6(sina/2)^2-8(sina/2)^4
化简:cos2a-6(sina/2)^2-8(sina/2)^4

化简:cos2a-6(sina/2)^2-8(sina/2)^4
cos2a-6(sina/2)^2-8(sina/2)^4
=cos2a-3(1-cosa)-2(1-cosa)^2
=cos2a-3+cosa-2+4cosa-2cos^2a
=cos2a-5+5cosa-(1+cos2a)
=cos2a-5+5cosa-1-cos2a
=5cosa-6

cos2a-6(sin(a/2))^2-8(sin(a/2))^4
=cos2a-5(1-cos2a)+4(sin(a/2))^2-8sin(a/2)^4
=6cos2a-5+4(sina/2)^2[1-2(sin(a/2))^2]
=6cos2a-5+2(1-cos2a)cos2a
=8cos2a-5-2(cos2a)^2