Question

7+7^2+7^3+...+7^100

Answer 1

Đặt A = 7 + 7² + 7³ + ... + 7¹⁰⁰

=> 7A = 7² + 7³ + 7⁴ + ... + 7¹⁰¹

=> 7A - A = ( 7² + 7³ + 7⁴ + ... + 7¹⁰¹ ) - ( 7 + 7² + 7³ + ... + 7¹⁰⁰ )

=> 6A = 7¹⁰¹ - 7

=> A = 7¹⁰¹ - 7/6

Answer 2

A= 1+7+7^2+7^3+.....+7^100

=) 7A= 7+7^2+7^3+7^4+.....+7^101

=)7A-A=6A=7^101-1