Question

[Quý Lương]

Tính: B = 7 - 7⁴ + 7⁷ - ... + 7³⁰¹

Answer 1

[Ngô Thế Trường ( CRIS DEVID )]

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

\(\Leftrightarrow7^3B=7^4-7^7+7^{10}+......+7^{304}\)

\(\Leftrightarrow7^3B-B=\left(7^4-7^7+7^{10}+...+7^{304}\right)-\left(7-7^4+...+7^{301}\right)\)

\(\Leftrightarrow342B=7^{304}-7\)

\(\Leftrightarrow\frac{7^{304}-7}{342}\)

Answer 2

[Kiên-Messi-8A-Boy2k6]

trường sai nhé

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

\(\Rightarrow7^3B=7^4-7^7+7^{10}-7^{13}+...+7^{302}\)

\(\Rightarrow7^3B+B=\left(7^4-7^7+7^{10}-7^{13}+...+7^{302}\right)+\left(7-7^4+7^{10}-7^{13}+....+7^{301}\right)\)

\(\Rightarrow344B=7^{302}+7-7^{301}\)

\(\Rightarrow B=\frac{7^{302}+7-7^{301}}{344}\)