Question

1+7+7^2+7^3+.....+7^49+7^50

Thực hiện phép tính giùm mik với !!!!!!!!!!!!!!!!!!!!

Answer 1

Đặt \(A=1+7+7^2+...7^{50}\)

\(7\cdot A=7+7^2+7^3+.....+7^{51}\)

\(7\cdot A-A=\left(7+7^2+7^3+.....+7^{51}\right)-\left(1+7+7^2+....+7^{50}\right)\)

\(A.\left(7-1\right)=\left(7-7\right)+\left(7^2-7^2\right)+.....+\left(7^{50}-7^{50}\right)+7^{51}-1\)

\(A\cdot6=7^{51}-1\Rightarrow A=\frac{7^{51}-1}{6}\)