Ta có: \(U=mn\left(m+n\right)+np\left(n+p\right)+pm\left(p+m\right)+2mnp\)
\(=mn\left(m+n\right)+np\left(n+p+m\right)+pm\left(p+m+n\right)\)
\(=mn\left(m+n\right)+p\left(n+p+m\right)\left(n+m\right)\)
\(=\left(n+m\right)\left(m+p\right)\left(n+p\right)\)