求值,4-1/4sin^22a-sin^2a-cos^4a

4-1/4sin^22a-sin^2a-cos^4a=4-1/4(2sinacosa)^2-sin^2a-cos^4a=4-sin^acos^2a-sin^2a-cos^4a=4-(1-cos^2a)cos^2a-sin^2a-cos^4a=4-cos^2a-sin^2a+cos^4a-cos^4a=4-(cos^2a+sin^2a)=4-1=3