2 câu trả lời
Bài giải :
|x+11×2|+|x+12×3|+...
Mà do vế trái luôn : x > 0
=>
x + 1/1xx2+ x + 2/3 +x+3/4+.....+x+1/99xx100=100x.
Vế trái có 99x
x = 1/1xx2 + 1/2xx3+1/3xx4+...+ 1/99xx100.
x = 1- 1/2 + 1/2 - 1/3 + 1/3 -1/4 +...+ 1/99-1/100.
x = 1 - 1/100.
x = 99/100.
|x+\dfrac{1}{1.2}| +|x+\dfrac{1}{2.3}|+...+|x+\dfrac{1}{99.100}|=100x(1)
Do \begin{cases} |x+\dfrac{1}{1.2}|\ge0\\|x+\dfrac{1}{2.3}|\ge0\\....\\|x+\dfrac{1}{99.10}|\ge0\end{cases}(∀x\in R)
\to |x+\dfrac{1}{1.2}| +|x+\dfrac{1}{2.3}|+...+|x+\dfrac{1}{99.100}|\ge 0 ∀x\in R
Do đó : 100x\ge 0\to x\ge 0
Khi đó (1) có dạng :
x+\dfrac{1}{1.2}+x+\dfrac{1}{2.3}+...+x+\dfrac{1}{99.100}=100x\\\to (x+x+...+x)+(\dfrac{1}{1.2}+\dfrac{1}{2.3}+...+\dfrac{1}{99.100})=100x\\\to 99x+(1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+...+\dfrac{1}{99}-\dfrac{1}{100})=100x\\\to 99x + (1-\dfrac{1}{100})=100x\\\to 99x + \dfrac{99}{100}=100x\\\to -x=\dfrac{-99}{100}\\\to x=\dfrac{99}{100}(Tm)
Vậy x=\dfrac{99}{100}