Áp dụng BĐT:Cho $$a,b$$ dương và $$x,y,z$$ dương với $$x+y+z=1$$. Chứng minh rằng:
$$\left(a+\dfrac{b}{x}\right)^4+\left(a+\dfrac{b}{y}\right)^4+\left(a+\dfrac{b}{z}\right)^4\geq 3\left(a+3b\right)^4$$
$$m^4+n^4+p^4 \geq \dfrac{\left(m+n+p\right)^4}{27}$$
Ta được:
$$\left(a+\dfrac{b}{x}\right)^4+\left(a+\dfrac{b}{y}\right)^4+\left(a+\dfrac{b}{z}\right)^4$$
$$\geq \dfrac{\left(3a+\dfrac{b}{x}+\dfrac{b}{y}+\dfrac{b}{z}\right)^4}{27}$$
$$\geq \dfrac{\left(3a+\dfrac{9b}{x+y+z}\right)^4}{27}$$
$$=\dfrac{\left(3a+9b\right)^4}{27}$$
$$=3\left(a+3b\right)^4$$