đặt \(P=\sqrt{\frac{\left(a+b\right)^3}{8ab\left(4a+4b+c\right)}}+\sqrt{\frac{\left(b+c\right)^3}{8bc\left(4b+4c+a\right)}}+\sqrt{\frac{\left(c+a\right)^3}{8ca\left(4c+4a+b\right)}}\)Q=8ab(4a+4b+c)+8bc(4b+4c+a)+8ca(4c+4a+b)=32(a+b+c)(ab+bc+ca)-72abcáp dụng holder ta có:\(P^2Q\ge8\left(a+b+c\right)^3\)theo schur thì \(\left(a+b+c\right)^3\ge4\left(a+b+c\right)\left(ab+bc+ca\right)-9abc\)\(\Rightarrow8\left(a+b+c\right)^3\ge32\left(a+b+c\right)\left(ab+bc+ca\right)-72abc\)\(\Rightarrow...
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đặt \(P=\sqrt{\frac{\left(a+b\right)^3}{8ab\left(4a+4b+c\right)}}+\sqrt{\frac{\left(b+c\right)^3}{8bc\left(4b+4c+a\right)}}+\sqrt{\frac{\left(c+a\right)^3}{8ca\left(4c+4a+b\right)}}\)
Q=8ab(4a+4b+c)+8bc(4b+4c+a)+8ca(4c+4a+b)
=32(a+b+c)(ab+bc+ca)-72abc
áp dụng holder ta có:
\(P^2Q\ge8\left(a+b+c\right)^3\)
theo schur thì \(\left(a+b+c\right)^3\ge4\left(a+b+c\right)\left(ab+bc+ca\right)-9abc\)
\(\Rightarrow8\left(a+b+c\right)^3\ge32\left(a+b+c\right)\left(ab+bc+ca\right)-72abc\)
\(\Rightarrow P^2\ge\frac{8\left(a+b+c\right)^3}{Q}\ge1\left(Q.E.D\right)\)
Cho xin đề :V