SAT Mathematics Practice Test 5

The sum of $\left({\mathit{a}}^2-1\right)$ and $\left(\mathit{a}+1\right)$ can be rewritten as (${\mathit{a}}^2-1)+\left(\mathit{a}+1\right),$ or ${\mathit{a}}^2-1+\mathit{a}+1,$ which is equal to ${\mathit{a}}^2+\mathit{a}+0.$ Therefore, the sum of the two expressions is equal to ${\mathit{a}}^2+\mathit{a}.$
Choices B and D are incorrect. Since neither of the two expressions has a term with ${\mathit{a}}^3$ the sum of the two expressions cannot have the term ${\mathit{a}}^3$ when simplified. Choice C is incorrect. This choice may result from mistakenly adding the terms ${\mathit{a}}^2$ and a to get $2{\mathit{a}}^2$