已知cos2a=3/5,求sin^4*a+cos^4*a=?
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已知cos2a=3/5,求sin^4*a+cos^4*a=?
cos2a=3/5
(sin2a)^2= (1-(cos2a)^2 = 16/25
cos^2a - sin^2a = 3/5
(cos^2a - sin^2a )^2 = 9/25
(cos^2a - sin^2a )^2 = 9/25
sin^4*a+cos^4*a-2(sinacosa)^2 = 9/25
sin^4*a+cos^4*a-1/2(sin2a)^2 = 9/25
sin^4*a+cos^4*a = 9/25+1/2(sin2a)^2=9/25+1/2*16/25=17/25
(sin2a)^2= (1-(cos2a)^2 = 16/25
cos^2a - sin^2a = 3/5
(cos^2a - sin^2a )^2 = 9/25
(cos^2a - sin^2a )^2 = 9/25
sin^4*a+cos^4*a-2(sinacosa)^2 = 9/25
sin^4*a+cos^4*a-1/2(sin2a)^2 = 9/25
sin^4*a+cos^4*a = 9/25+1/2(sin2a)^2=9/25+1/2*16/25=17/25
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