Với `x \ne -10,x \ne -2` có:
`A=[x^2-1]/[x+10].[2x]/[x+2]+[x^2-1]/[x+10].[10-x]/[x+2]`
`A=[2x(x^2-1)+(x^2-1)(10-x)]/[(x+10)(x+2)]`
`A=[(x^2-1)(2x+10-x)]/[(x+10)(x+2)]`
`A=[(x^2-1)(x+10)]/[(x+10)(x+2)]`
`A=[x^2-1]/[x+2]`
\(A=\dfrac{x^2-1}{x+10}.\dfrac{2x}{x+2}+\dfrac{x^2-1}{x+10}.\dfrac{10-x}{x+2}\)
\(=\dfrac{\left(x^2-1\right)2x}{\left(x+10\right)\left(x+2\right)}+\dfrac{\left(x^2-1\right)\left(10-x\right)}{\left(x+10\right)\left(x+2\right)}\)
\(=\dfrac{\left(x^2-1\right)2x+\left(x^2-1\right)\left(10-x\right)}{\left(x+10\right)\left(x+2\right)}\)
\(=\dfrac{\left(x^2-1\right)\left(x+10\right)}{\left(x+10\right)\left(x+2\right)}\) \(=\dfrac{x^2-1}{x+2}\)
