Chứng minh a, 1+tan^2 a=1/cos^2 a b, cotan^2a - cos^2a=cotan^2a.cos^2a
2 câu trả lời
a,
$VT=1+\dfrac{\sin^2a}{\cos^2a}$
$=\dfrac{\cos^2a+\sin^2a}{\cos^2a}$
$=\dfrac{1}{\cos^2a}$
$= VP$
b,
$VT=\cot^2a-\cos^2a$
$=\dfrac{\cos^2a-\sin^2a.\cos^2a}{\sin^2a}$
$=\dfrac{\cos^2a(1-\sin^2a)}{\sin^2a}$
$=\dfrac{\cos^2a.\cos^2a}{\sin^2a}$
$=\cos^2a.\cot^2a$
$= VP$
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