Tính tổng Sn = 1 + 2a + 3a^2 + 4a^3 + ... + (n + 1)a^n (a khác 1 là số cho trước)

A. $\frac{\left(\mathrm{n}+1\right){\mathrm{a}}^{\mathrm{n}+2}-\left(\mathrm{n}+2\right){\mathrm{a}}^{\mathrm{n}+1}+1}{{\left(1-\mathrm{a}\right)}^2}$

B. $\frac{\left(\mathrm{n}+1\right){\mathrm{a}}^{\mathrm{n}+2}+\left(\mathrm{n}+2\right){\mathrm{a}}^{\mathrm{n}+1}+1}{{\left(1-\mathrm{a}\right)}^2}$

C. $\frac{\left(\mathrm{n}+1\right){\mathrm{a}}^{\mathrm{n}+2}-\left(\mathrm{n}+2\right){\mathrm{a}}^{\mathrm{n}+1}-1}{{\left(1-\mathrm{a}\right)}^2}$

D. $\frac{\left(\mathrm{n}+1\right){\mathrm{a}}^{\mathrm{n}+2}+\left(\mathrm{n}+2\right){\mathrm{a}}^{\mathrm{n}+1}-1}{{\left(1-\mathrm{a}\right)}^2}$