## 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

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

## Answers ( )

1. 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

2. Đáp án:

Giải thích các bước giải:

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