M = x+y/z + x+z/y + y+z/x

M = x+y+z/z + x+y+z/y + x+y+z/x - z/z - y/y - x/x

M = (x+y+z).(1/z + 1/y + 1/x) - 1 - 1 - 1

M = 2020.1/202 - 3

M = 10 - 3 = 7

\(M=\frac{x+y}{z}+\frac{x+z}{y}+\frac{y+z}{x}\\ M=\frac{x+y+z}{z}+\frac{x+y+z}{y}+\frac{x+y+z}{x}-\frac{z}{z}-\frac{y}{y}-\frac{x}{x}\\ M=\left(x+y+z\right)\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)-1-1-1\\ M=2020\cdot\frac{1}{202}-3\\ M=10-3=7\)

Ta có : M = \(\frac{x+y}{z}+\frac{x+z}{y}=\frac{y+z}{x}\)

\(\Rightarrow M+3=\left(\frac{x+y}{z}+1\right)+\left(\frac{x+z}{y}+1\right)+\left(\frac{y+z}{x}+1\right)\)

\(\Rightarrow M+3=\frac{x+y+z}{z}+\frac{x+y+z}{y}+\frac{x+y+z}{x}\)

\(\Rightarrow M+3=\left(x+y+z\right).\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)\)

\(\Rightarrow M+3=2020.\frac{1}{202}\)

=> M + 3 = 10

=> M = 7

Vậy M = 7

b) Ta có : \(A=\frac{2}{3^2}+\frac{2}{5^2}+\frac{2}{7^2}+...+\frac{2}{2017^2}\)

\(=\frac{2}{3.3}+\frac{2}{5.5}+\frac{2}{7.7}+...+\frac{2}{2017.2017}\)

\(< \frac{2}{\left(3+1\right)\left(3-1\right)}+\frac{2}{\left(5-1\right)\left(5+1\right)}+\frac{2}{\left(7-1\right)\left(7+1\right)}+...+\frac{2}{\left(2017-1\right)\left(2016-1\right)}\)

\(=\frac{2}{2.4}+\frac{2}{4.6}+\frac{2}{6.8}+...+\frac{2}{2016.2018}\)

\(=\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+\frac{1}{6}-\frac{1}{8}+...+\frac{1}{2016}-\frac{1}{2018}\)

\(=\frac{1}{2}-\frac{1}{2018}\)

\(=\frac{1008}{2018}=\frac{504}{1009}\)

=> \(A< \frac{504}{1009}\left(\text{ĐPCM}\right)\)

Bài a ) \(\frac{2}{\left(3+1\right)\left(3-1\right)}\) là như thế nào bạn

tắt nhé

Đừng làm tắt nhé

\(M=\frac{x+y}{z}+\frac{x+z}{y}+\frac{y+z}{x}\\ M=\frac{x+y+z}{z}+\frac{x+y+z}{y}+\frac{x+y+z}{x}-\frac{z}{z}-\frac{y}{y}-\frac{x}{x}\\ M=\left(x+y+z\right).\left(\frac{1}{y}+\frac{1}{x}+\frac{1}{z}\right)-1-1-1\\ M=2020.\frac{1}{202}-3\\ M=10-3\\ M=7\)

Sửa lại đề là tính \(M=\frac{x+y}{z}+\frac{x+z}{y}+\frac{y+z}{x}\) nhé.

Ta có:

\(M=\frac{x+y}{z}+\frac{x+z}{y}+\frac{y+z}{x}.\)

\(\Rightarrow M=\frac{x+y+z}{z}+\frac{x+y+z}{y}+\frac{x+y+z}{x}-\frac{z}{z}-\frac{y}{y}-\frac{x}{x}\)

\(\Rightarrow M=\left(x+y+z\right).\left(\frac{1}{z}+\frac{1}{y}+\frac{1}{x}\right)-1-1-1\)

Mà \(x+y+z=2020;\frac{1}{x}+\frac{1}{y}+\frac{1}{z}=\frac{1}{202}.\)

\(\Rightarrow M=2020.\frac{1}{202}-3\)

\(\Rightarrow M=10-3\)

\(\Rightarrow M=7\)

Vậy \(M=7.\)

Chúc bạn học tốt!

Các bạn ơi cô mình giảng hình như thế này :
Ta có : x+y+z =2020

x + y = 2020 -y

x+z =2020 - y

y+z =2020 -z

Ta có :

\(\frac{2020-z}{z}\) = ...

Như thế đó maong các bạn giải như thế