giúp mk với a |x|+1/3|+1/3|+1/3|=0 b 2/6+2/12+2/20+…+2/x(x+1)=4/5

Question

giúp mk với

a |x|+1/3|+1/3|+1/3|=0
b 2/6+2/12+2/20+…+2/x(x+1)=4/5

in progress 0
Lydia 3 tuần 2021-08-21T17:14:44+00:00 2 Answers 0 views 0

Answers ( )

    0
    2021-08-21T17:15:52+00:00

    Giải thích các bước giải:

     a) `|x|+1/3+1/3+1/3=0`

    `=>|x|+1=0`

    `=>|x|=-1` $\text{(loại vì }|x|\geq0)$

     b) `2/6+2/12+2/20+…+2/(x(x+1))=4/5`

    `=>2(1/6+1/12+1/20+…+1/(x(x+1)))=2. 2/5`

    `=>1/6+1/12+1/20+…+1/(x(x+1))=2/5`

    `=>1/2.3+1/3.4+1/4.5+…+1/(x(x+1))=2/5`

    `=>1/2-1/3+1/3-1/4+1/4-1/5+…+1/x-1/(x+1)=2/5`

    `=>1/2-1/(x+1)=2/5`

    `=>1/(x+1)=1/2-2/5=1/10`

    `=>x+1=10`

    `=>x=9`

    0
    2021-08-21T17:16:42+00:00

    Đáp án: 

     

    Giải thích các bước giải:

    $a) |||x|+\frac{1}{3}|+\frac{1}{3}|+\frac{1}{3}|=0 \\ 
    \Leftrightarrow ||x|+\frac{1}{3}|+\frac{1}{3}| = \frac{-1}{3}(vô lí)
    \\
    b)  Pt \Leftrightarrow \frac{1}{2.3} + \frac{1}{3.4} + … + \frac{1}{x(x+1)} = \frac{2}{5}\\
    \Leftrightarrow \frac{1}{2} -\frac{1}{3}+ \frac{1}{3} – \frac{1}{4} + … + \frac{1}{x} – \frac{1}{x+1} = \frac{2}{5}\\
    \Leftrightarrow \frac{1}{2} – \frac{1}{x+1} = \frac{2}{5}\\
    \Leftrightarrow \frac{1}{x+1} = \frac{1}{10}
    \Leftrightarrow  x + 1 = 10 \Leftrightarrow x = 9
    $

Leave an answer

Browse

35:5x4+1-9:3 = ? ( )