Question

Tính:

\(B=7^7-7^8+7^9-7^{10}+...+7^{2015}\)

Answer 1

B = 7^7 - 7^8 + 7^9 -7^10 +....+7^2015

=> 7B = 7^8 -7^9 +....+ 7^2014-7^2015 + 7^2016

=> 8B = 7^7 + 7^2016 

=> B = (7^7 + 7^2016)/8

Answer 2

B=7⁷-7⁸+7⁹-7¹⁰+...+7²⁰¹⁴-7²⁰¹⁵

=>7B=7⁸-7⁹+7¹⁰-7¹¹+...+7²⁰¹⁵-7²⁰¹⁶

=>B+7B=(7⁷-7⁸+7⁹-7¹⁰+...+7²⁰¹⁴-7²⁰¹⁵)+(7⁸-7⁹+7¹⁰-7¹¹+...+7²⁰¹⁵-7²⁰¹⁶)

=>8B=7⁷+7²⁰¹⁶

=>B=\(\frac{7^7+7^{2016}}{8}\)