Question

c/m:   1/7² -1/7⁴+...+1/7^4x-2 -1/7^4x+... + 1/7⁹⁸  - 1/7¹⁰⁰<1/50

Answer 1

GIUP MINH LAM BAI NAY VOI

Answer 2

Gọi A=\(\frac{1}{7^2}-\frac{1}{7^4}+\frac{1}{7^6}-\frac{1}{7^8}+...+\frac{1}{7^{98}}-\frac{1}{7^{100}}\)

Nhân \(\frac{1}{7^2}\)vào A ta được

\(\frac{1}{7^2}\).A=   \(\frac{1}{7^4}-\frac{1}{7^6}+\frac{1}{7^8}-...-\frac{1}{7^{98}}+\frac{1}{7^{100}}+\frac{1}{7^{102}}\)

     A=\(\frac{1}{7^2}-\frac{1}{7^4}+\frac{1}{7^6}-\frac{1}{7^8}+....+\frac{1}{7^{98}}-\frac{1}{7^{100}}\)

Cộng \(\frac{1}{7^2}A\)+\(A\)=\(\frac{1}{49}-\frac{1}{7^{102}}\)\(\Rightarrow\frac{50}{49}A=\frac{1}{49}-\frac{1}{7^{102}}\Rightarrow A=\left(\frac{1}{49}-\frac{1}{7^{102}}\right).\frac{49}{50}\)

\(A=\frac{1}{50}-\frac{1}{7^{102}}.\frac{49}{50}

Answer 3

bài này là bài lớp 7 mới học nè