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vansang02121998

$A=sin \ a\sqrt{1+cos^2 a}+cos \ a\sqrt{1+\sin^2 a}$

Áp dụng Bunhia Copxki

$A^2 \le (sin^2a+cos^2a)(2+sin^2a+cos^2a)$

mà $sin^2a+cos^2a=1 \Rightarrow A^2 \le 3 \Rightarrow A \le \sqrt{3}$

Dấu $"="$ xảy ra khi $a=45^o$
 
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