Cho $x,y\geq 1$
CMR: $\frac{1}{1+x^2}+\frac{1}{1+y^2}\geq \frac{2}{1+xy}$
BĐT <=> $\frac{1}{1+x^{2}}-\frac{1}{1+xy}+\frac{1}{1+y^{2}}-\frac{1}{1+xy}\geq 0<=>\frac{xy-x^{2}}{(1+x^{2})(1+xy)}+\frac{xy-y^{2}}{(1+y^{2})(1+xy)}\geq 0<=>\frac{(1+y^{2})(xy-x^{2})+(1+x^{2})(xy-y^{2})}{(1+x^{2})(1+y^{2})(1+xy)}\geq 0<=>\frac{(y-x)^{2}(xy-1)}{(1+x^{2})(1+y^{2})(1+xy)}\geq 0$ (đúng)
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