5.6 x 2 + 2.8 x 8 + 11.2 x 2- 48.5

= 2.8 x 2 x 2 + 2.8 x 8 + 2.8 x 4 x2 - 48.5

= 2.8 x 4 + 2.8 x 8 + 2.8 x 8 - 48.5

= 2.8 x (4+8+8) - 48.5

= 2.8 x 20 - 48.5

= 560 - 48.5

= 511.5 

|5.6 - \(x\)| = 4.6

|30 - \(x\)| = 24

\(x< 30\) ⇒ 30 - \(x\) = 24 ⇒ \(x\) =  30 - 24  = -6

\(x>30\) ⇒ -(30 - \(x\)) = 24 ⇒ -30 + \(x\) = 24   ⇒ \(x\) = 24 + 30 = 54

Vậy \(x\) \(\in\) {  -6; 54}

`@` `\text {Ans}`

`\downarrow`

\(\left|5\cdot6-x\right|=4\cdot6\)

`\Rightarrow`\(\left|5\cdot6-x\right|=24\)

`\Rightarrow`\(\left[{}\begin{matrix}5\cdot6-x=24\\5\cdot6-x=-24\end{matrix}\right.\)

`\Rightarrow`\(\left[{}\begin{matrix}30-x=24\\30-x=-24\end{matrix}\right.\)

`\Rightarrow`\(\left[{}\begin{matrix}x=30-24\\x=30-\left(-24\right)\end{matrix}\right.\)

`\Rightarrow`\(\left[{}\begin{matrix}x=6\\x=54\end{matrix}\right.\)

Vậy, `x={6; 54}.`

\(\frac{1}{3.4}+\frac{1}{3.4}+...+\frac{1}{x\left(x+1\right)}=\frac{1}{3}-\frac{1}{x+1}\)

\(\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+...+\frac{1}{x\left(x+1\right)}\)

\(=\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+.....+\frac{1}{x}-\frac{1}{x+1}\)

\(=\frac{1}{3}-\frac{1}{x+1}\)

(1/3-1/4+1/4-1/5+1/5-.......+1/x.(x+1)=3/10

1/3-1/x+1=3/10

tự làm...

1/3.4+1/4.5+1/5.6+1/6.7+....+1/x(x+1)=3/10

<=> \(\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{\left(x+1\right)x}=\frac{1}{3}-\frac{1}{x+1}=\frac{3}{10}\)

<=> \(\frac{1}{x+1}=\frac{1}{3}-\frac{3}{10}=\frac{1}{30}\)=> x+1=30=>x=29

\(\frac{1}{3\cdot4}+\frac{1}{4\cdot5}+\frac{1}{5\cdot6}+\frac{1}{6\cdot7}+...+\frac{1}{x\left(x+1\right)}=\frac{3}{10}\)

\(\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{x}-\frac{1}{x+1}=\frac{3}{10}\)

\(\frac{1}{3}-\frac{1}{x+1}=\frac{3}{10}\)

\(\frac{1}{x+1}=\frac{1}{3}-\frac{3}{10}\)

\(\frac{1}{x+1}=\frac{1}{30}\)

\(\Rightarrow x+1=30\)

\(x=30-1=29\)

nnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn

bài này có x mà ko có vế phải à

chịu, cô tui cho đề thế mà

\(A=\left|3,7-x\right|+2,5\ge2,5\)

\(MinA=2,5\Leftrightarrow3,7-x=0\Rightarrow x=3,7\)

\(B=\left|x+1,5\right|+4,5\ge4,5\)

\(MinB=4,5\Leftrightarrow x+1,5=0\Rightarrow x=-1,5\)

.3-2/2.3 + 4-3/3.4 + 5-4/4.5 + 6-5/5.6 +...+ 20-19/19.20=18/x

1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 +...+ 1/19 - 1/20=18/x

1/2 - 1/20=18/x

10/20 - 1/20=18/x

9/20=18/x

18/40=18/x

=>x=40

Vậy x=40

Thanks

3.(1/4.5+1/5.6+...+1/9.10).x=9/2

3.(1/4-1/5+1/5-1/6+...+1/9-1/10).x=9/2

3.(1/4-1/10).x=9/2

3.3/20.x=9/2

9/20.x=9/2

x=9/2:9/20

x=10