Cho a,b,c>0 .Chứng minh:$\sqrt{\frac{2a}{b+c}}+\sqrt{\frac{2b}{c+a}}+\sqrt{\frac{2c}{a+b}}\leq 3$
Ta sử dụng $ Cauchy-Schwarz$ dưới dạng:
$$\sqrt{Ax}+\sqrt{By}+\sqrt{Cz}\leq \sqrt{(A+B+C)(x+y+z)}\tag{1}.$$
Ta viết:
$$\sqrt{\frac{2a}{b+c}}+\sqrt{\frac{2b}{c+a}}+\sqrt{\frac{2c}{a+b}}=\frac{\sqrt{2a(c+a)(a+b)}+\sqrt{2b(b+c)(a+b)}+\sqrt{2c(b+c)(c+a)}}{\sqrt{(a+b)(b+c)(c+a)}}.$$
Áp dung $(1)$ với:
$$A=2a(c+a) \ \text{and} \ x=a+b \\ B=2b(a+b) \ \text{and} \ y=b+c \\ C=2c(b+c) \ \text{and} \ z=c+a $$