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Posted: **Thu Feb 19, 2009 9:43 am**

Aratati ca
\( \frac{1}{sin^4x}+\frac{1}{cos^4x}\ge 8 \) \( (\forall)x\neq k\cdot \frac{\pi}{2} \)

Posted: **Thu Feb 19, 2009 10:44 am**

\( \frac{1}{x^4}+\frac{1}{y^4}\geq \frac{4}{x^4+y^4}\geq\frac{8}{(x^2+y^2)^2}=8. \)

Inegalitate cu sin x , cos x