Bài 6:
a: \(\left(1-\cos\alpha\right)\left(1+\cos\alpha\right)=1-\cos^2\alpha=\sin^2\alpha\)
b: \(1+\sin^2\alpha+\cos^2\alpha=1+1=2\)
c: \(\sin\alpha-\sin\alpha\cdot\cos^2\alpha\)
\(=\sin\alpha\left(1-\cos^2\alpha\right)\)
\(=\sin\alpha\cdot\sin^2\alpha\)
\(=\sin^3\alpha\)
d: Ta có: \(\sin^4\alpha+2\cdot\sin^2\alpha\cdot\cos^2\alpha+\cos^4\alpha\)
\(=\left(\sin^2\alpha+\cos^2\alpha\right)^2\)
=1
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