Question

cho a,b,c>0 thỏa a+b+c=3

chứng minh $\frac{a^3}{b\left(2c+a\right)}+\frac{b^3}{c\left(2a+b\right)}+\frac{c^3}{a\left(2b+c\right)}\ge1$a3b(2c+a) +b3c(2a+b) +c3a(2b+c) ≥1

Answer 1

Theo bất đẳng thức Bunhiacopxki thì

\(\left(ab(2c+a)+bc(2a+b)+ca(2b+c)\right)\left(\dfrac{a^4}{ab(2c+a)}+\dfrac{b^4}{bc(2a+b)}+\dfrac{c^4}{ca(2b+c)}\right)\geq (a^2+b^2+c^2)^2\)

Do đó \(VT\geq \dfrac{(a^2+b^2+c^2)^2}{a^2b+b^2c+c^2a+6abc}\)

Ta có \(3=a+b+c\geq 3\sqrt[3]{abc}, 3(a^2+b^2+c^2)\geq (a+b+c)^2\)

và \(2a^2b\leq a^2b^2+a^2,...\Rightarrow 2(a^2b+b^2c+c^2a)\leq a^2b^2+b^2c^2+c^2a^2+(a^2+b^2+c^2)\)

Mà \(3(a^2b^2+b^2c^2+c^2a^2)\leq (a^2+b^2+c^2)^2\) và \(3(a^2+b^2+c^2)\leq (a^2+b^2+c^2)^2\)

nên ta suy ra đpcm