Question

tinh

B=7-7⁴+7⁷-...+7³⁰¹

Answer 1

7³B=7⁴-7⁷⁺7¹⁰-...-7³⁰¹+7³⁰⁴

7³B-B=(7⁴-7⁷+7¹⁰-...-7³⁰¹+7³⁰⁴)-(7-7⁴+7⁷-...+7³⁰¹)

343B-B=7⁴-7⁷+7¹⁰-...-7³⁰¹+7³⁰⁴-7+7⁴-7⁷+...-7³⁰¹

342B=7³⁰⁴-7

B=\(\frac{7^{304}-7}{342}\)

Answer 2

lay may tinh ma tinh hi

Answer 3

\(B=7-7^4+7^7-...+7^{301}\)

\(7^3B=7^4-7^7+7^{10}-...-7^{301}+7^{302}\)

\(344B=7^3B+B=7+7^{302}\)

\(\Rightarrow B=\frac{7+7^{302}}{344}\)