\(A=3\left[\left(sin^2a+cos^2a\right)^2-2sin^2a.cos^2a\right]-2\left[\left(sin^2a+cos^2a\right)^2-3sin^2a.cos^2a\left(sin^2a+cos^2a\right)\right]\)
\(=3\left(1-2sin^2a.cos^2a\right)-2\left(1-3sin^2a.cos^2a\right)\)
\(=3-6sin^2a.cos^2a-2+6.sin^2a.cos^2a\)
\(=1\)
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