b) nhân 2 vế với 2 cho dễ làm

\(\Leftrightarrow\frac{2}{3}x+\frac{1}{3}x=\frac{7}{8}-\frac{7}{10}\)

\(x=\frac{7}{40}\)

\(A=\left(y^2+2y\left(x+1\right)+\left(x+1\right)^2\right)+\left(2x^2-2x+2-\left(x+1\right)^2\right)\)

\(=\left(y+x+1\right)^2+\left(x-2\right)^2-3\ge-3\)

Min A=-3 khi x=2;y=-3

2) A= \(n^2\left(n+1\right)+2n\left(n+1\right)=n\left(n+1\right)\left(n+2\right)\)

A là tích 3 số TN liên tiep => A\(⋮\)2; A\(⋮\)3

=> A\(⋮\)2.3

A\(⋮\)6

1)  \(55^{n+1}-55^n=55^n\left(55-1\right)=55^n.54⋮54\)

4851 sao ban dat nhiu cau hoi zay???