\(D=\left(-\dfrac{1}{7}\right)^0+\left(-\dfrac{1}{7}\right)^1+...+\left(-\dfrac{1}{7}\right)^{2017}\)
=>\(-\dfrac{1}{7}\cdot D=\left(-\dfrac{1}{7}\right)^1+\left(-\dfrac{1}{7}\right)^2+...+\left(-\dfrac{1}{7}\right)^{2018}\)
=>\(-\dfrac{1}{7}D-D=\left(-\dfrac{1}{7}\right)^1+\left(-\dfrac{1}{7}\right)^2+...+\left(-\dfrac{1}{7}\right)^{2018}-1-\left(-\dfrac{1}{7}\right)^1-...-\left(-\dfrac{1}{7}\right)^{2017}\)
=>\(-\dfrac{8}{7}\cdot D=\left(-\dfrac{1}{7}\right)^{2018}-1=\dfrac{1}{7^{2018}}-1\)
=>\(D\cdot\dfrac{8}{7}=1-\dfrac{1}{7^{2018}}=\dfrac{7^{2018}-1}{7^{2018}}\)
=>\(D=\dfrac{7^{2018}-1}{7^{2018}}:\dfrac{8}{7}=\dfrac{7^{2018}-1}{8\cdot7^{2017}}\)
=>Không có câu nào đúng
