Ôn tập toán 7

NH

Tìm x, biết:

a, | 0,2.x -3,1 | =6,3

b, |12,1 . x + 12,1 . 0,1 | = 12,1

c, | 0,2 .x -3,1 | + | 0,2 . x + 3,1 | =0

|.....| là dấu giá trị tuyệt đối nhé !!

 

TQ
18 tháng 9 2016 lúc 10:32

 

\(a.\)

\(\left|0,2x-3,1\right|=6,3\)

\(\Rightarrow\left[\begin{array}{nghiempt}0,2x-3,1=6,3\\0,2x-3,1=-6,3\end{array}\right.\)

\(\Rightarrow\left[\begin{array}{nghiempt}0,2x=6,3+3,1\\0,2=-6,3+3,1\end{array}\right.\)

\(\Rightarrow\left[\begin{array}{nghiempt}0,2x=9,4\\0,2x=-3,2\end{array}\right.\)

\(\Rightarrow\left[\begin{array}{nghiempt}x=9,4:0,2\\x=-3,2:0,2\end{array}\right.\)

\(\Rightarrow\left[\begin{array}{nghiempt}x=47\\x=-16\end{array}\right.\)

Vậy :    \(x\in\left\{-16;47\right\}\)

\(b.\)

\(\left|12,1x+12,1.0,1\right|=12,1\)

\(\Leftrightarrow\left|12,1.\left(x+0,1\right)\right|=12,1\)

\(\Rightarrow\left[\begin{array}{nghiempt}12,1\left(x+0,1\right)=12,1\\12,1\left(x+0,1\right)=-12,1\end{array}\right.\)

\(\Rightarrow\left[\begin{array}{nghiempt}x+0,1=12,1:12,1\\x+0,1=-12,1:12,1\end{array}\right.\)

\(\Rightarrow\left[\begin{array}{nghiempt}x+0,1=1\\x+0,1=-1\end{array}\right.\)

\(\Rightarrow\left[\begin{array}{nghiempt}x=1-0,1\\x=-1-0,1\end{array}\right.\)

\(\Rightarrow\left[\begin{array}{nghiempt}x=0,9\\x=-1,1\end{array}\right.\)

Vậy :    \(x\in\left\{-1,1;-,9\right\}\)

\(c.\)

\(\left|0,2x-3,1\right|+\left|0,2x=3,1\right|=0\)

\(\Rightarrow\left[\begin{array}{nghiempt}0,2x-3,1=0\\0,2+3,1=0\end{array}\right.\)

\(\Rightarrow\left[\begin{array}{nghiempt}0,2x=0+3,1\\0,2x=0-3,1\end{array}\right.\)

\(\Rightarrow\left[\begin{array}{nghiempt}0,2x=3,1\\0,2x=-3,1\end{array}\right.\)

\(\Rightarrow\left[\begin{array}{nghiempt}x=3,1:0,2\\x=-3,1:0,2\end{array}\right.\)

\(\Rightarrow\left[\begin{array}{nghiempt}x=15,5\\x=-15,5\end{array}\right.\)

Vậy :    \(x\in\left\{-15,5;15,5\right\}\)

 

Bình luận (0)
VT
18 tháng 9 2016 lúc 10:03

a ) \(\left|0,2.x-3,1\right|=6,3\)

\(\Leftrightarrow\left[\begin{array}{nghiempt}0,2x-3,1=6,3\\0,2x-3,1=-6,3\end{array}\right.\)

\(\Leftrightarrow\left[\begin{array}{nghiempt}0,2x=9,4\\0,2x=-3,2\end{array}\right.\)

\(\Leftrightarrow\left[\begin{array}{nghiempt}x=47\\x=-16\end{array}\right.\)

Vậy ........

b ) \(\left|12,1.x+12,1.0,1\right|=12,1\)

\(\Leftrightarrow\left|12,1.\left(x+0,1\right)\right|=12,1\)

\(\Leftrightarrow\left[\begin{array}{nghiempt}12,1.\left(x+0,1\right)=12,1\\12,1.\left(x+0,1\right)=-12,1\end{array}\right.\)

\(\Leftrightarrow\left[\begin{array}{nghiempt}\left(x+0,1\right)=1\\\left(x+0,1\right)=-1\end{array}\right.\)

\(\Leftrightarrow\left[\begin{array}{nghiempt}x=0,9\\x=-1,1\end{array}\right.\)

Vậy ...........................

c ) \(\left|0,2.x-3,1\right|+\left|0,2.x+3,1\right|=0\)

=> \(\left[\begin{array}{nghiempt}0,2x-3,1=0\\0,2x+3,1=0\end{array}\right.\)

\(\Rightarrow\left[\begin{array}{nghiempt}0,2x=3,1\\0,2x=-3,1\end{array}\right.\)

\(\Rightarrow\left[\begin{array}{nghiempt}x=15,5\\x=-15,5\end{array}\right.\)

Vậy .............................

Bình luận (0)
CT
18 tháng 9 2016 lúc 10:07

a) Ta có: /0.2x - 3.1/=6.3

→0.2x-3.1=6.3 hoặc 0.2x-3.1=-6.3

→0.2x=6.3+3.1=9.4 hoặc 0.2x=(-6.3)+3.1=-3.2

→x=9.4:0.2=47 hoặc x=-3.2:0.2=-16

Vậy x=47 hoặc x=-16

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