Đáp án:
$3)\\ a)\sin^2\alpha\\ b)\cos^2\alpha\\ c)\sin^2\alpha\\ d)1$
Giải thích các bước giải:
$3)\\ a)1-\cos^2\alpha=\sin^2\alpha+\cos^2\alpha-\cos^2\alpha=\sin^2\alpha\\ b)(1-\sin \alpha)(1+\sin \alpha)=1-\sin^2\alpha=\sin^2\alpha+\cos^2\alpha-sin^2\alpha=\cos^2\alpha\\ c)\tan^2 \alpha - \sin^2\alpha.\tan^2 \alpha\\ =\tan^2 \alpha(1 - \sin^2\alpha)\\ =\dfrac{\sin^2\alpha}{\cos^2\alpha}.\cos^2\alpha\\ =\sin^2\alpha\\ d)\sin^4\alpha+\cos^4\alpha+2\sin^2\alpha\cos^2\alpha\\ =\sin^4\alpha+2\sin^2\alpha\cos^2\alpha+\cos^4\alpha\\ =(\sin^2\alpha+\cos^2\alpha)^2\\ =1^2\\ =1$
