(3/8+-3/4+7/12):5/6+1/2
= 5/24:5/6+1/2

= 1/4 + 1/2

=3/4

=5/24 : 5/6 +1/2

=1/4 + 1/2

= 3/4

\(E=2.3+3.4+4.5+3.6+2.7+4.15=2\left(3+7\right)+3\left(4+6\right)+4\left(5+15\right)=2.10+3.10+4.20=20+30+80=130\)

\(F=3\left(12+13+14+15\right)+3\left(8+7+6+5\right)=3\left(12+8+13+7+14+6+15+5\right)=3\left(20+20+20+20\right)=3.80=240\)

f: Ta có: \(E=3\cdot\left(12+13+14+15\right)+3\left(8+7+6+5\right)\)

\(=3\left(12+13+14+15+8+7+6+5\right)\)

\(=3\cdot80=240\)

ta có A=\(\dfrac{6}{8}\)+\(\dfrac{6}{56}\)+\(\dfrac{6}{140}\)+...+\(\dfrac{6}{1100}\)+\(\dfrac{6}{1400}\)

           =\(\dfrac{3}{4}\)+\(\dfrac{3}{28}\)+\(\dfrac{3}{70}\)+...+\(\dfrac{3}{550}\)+\(\dfrac{3}{700}\)

           =\(\dfrac{3}{1.4}\)+\(\dfrac{3}{4.7}\)+\(\dfrac{3}{7.10}\)+...+\(\dfrac{3}{22.25}\)+\(\dfrac{3}{25.28}\)

           =1-\(\dfrac{1}{4}\)+\(\dfrac{1}{4}\)-\(\dfrac{1}{7}\)+\(\dfrac{1}{7}\)-\(\dfrac{1}{10}\)+...+\(\dfrac{1}{22}\)-\(\dfrac{1}{25}\)+\(\dfrac{1}{25}\)-\(\dfrac{1}{28}\)

          =1-\(\dfrac{1}{28}\)

          =\(\dfrac{27}{28}\)

Vậy A=\(\dfrac{27}{28}\)

Ta có:

Ta có :

A = 6/8 + 6/56 + 6/140 +...+ 6/1100 + 6/1400

A= 3/4 + 3/28 + 3/70 +...+ 3/550 + 3/700 

A= 3/1.4 + 3/4.7 + 3/ 7.10 + 3/22.25 + 3/25.28

A= 1- 1/28

A= 27/28

a: \(\dfrac{6}{5\sqrt{8}}=\dfrac{6}{10\sqrt{2}}=\dfrac{3}{5\sqrt{2}}=\dfrac{3\sqrt{2}}{10}\)

b: \(\dfrac{7}{5+2\sqrt{3}}=\dfrac{7\left(5-2\sqrt{3}\right)}{13}\)

c: \(\dfrac{6}{\sqrt{7}-\sqrt{5}}=\dfrac{6\left(\sqrt{7}+\sqrt{5}\right)}{2}=3\left(\sqrt{7}+\sqrt{5}\right)\)

a) \(\dfrac{6}{5\sqrt{8}}\)

\(=\dfrac{6}{5\cdot2\sqrt{2}}\)

\(=\dfrac{6}{10\sqrt{2}}\)

\(=\dfrac{3\sqrt{2}}{5\sqrt{2}\cdot\sqrt{2}}\)

\(=\dfrac{3\sqrt{2}}{10}\)

b) \(\dfrac{7}{5+2\sqrt{3}}\)

\(=\dfrac{7\left(5-2\sqrt{3}\right)}{\left(5+2\sqrt{3}\right)\left(5-2\sqrt{3}\right)}\)

\(=\dfrac{7\left(5-2\sqrt{3}\right)}{5^2-\left(2\sqrt{3}\right)^2}\)

\(=\dfrac{7\left(5-2\sqrt{3}\right)}{13}\)

\(=\dfrac{35-14\sqrt{3}}{13}\)

c) \(\dfrac{6}{\sqrt{7}-\sqrt{5}}\)

\(=\dfrac{6\left(\sqrt{7}+\sqrt{5}\right)}{\left(\sqrt{7}+\sqrt{5}\right)\left(\sqrt{7}-\sqrt{5}\right)}\)

\(=\dfrac{6\left(\sqrt{7}+\sqrt{5}\right)}{2}\)

\(=3\sqrt{7}+3\sqrt{5}\)

c: \(\Leftrightarrow x+7=38\)

hay x=31

15-(2x-3)=6                        (x-2).3=3²

⇔   2x-3=9                      ⇔(x-2).3=9

  ⇔  2x=12                      ⇔x-2=3

    ⇔   x=6                         ⇔x=5

(x+7)-25=13                  450:(x-19)=50

⇔x+7=38                       ⇔x-19=9

⇔x=31                             ⇔x=28

a: Ta có: \(\dfrac{x+6}{8}+\dfrac{x+8}{6}+\dfrac{x+1}{13}+3=0\)

\(\Leftrightarrow\dfrac{x+14}{6}+\dfrac{x+14}{6}+\dfrac{x+14}{13}=0\)

\(\Leftrightarrow x+14=0\)

hay x=-14

b) Ta có: \(\dfrac{x-5}{10}+\dfrac{x-7}{8}+\dfrac{x-1}{14}=3\)

\(\Leftrightarrow\dfrac{x-15}{10}+\dfrac{x-15}{8}+\dfrac{x-15}{14}=0\)

\(\Leftrightarrow x-15=0\)

hay x=15

a: \(\dfrac{x}{6}=\dfrac{8}{3}\)

=>\(x=6\cdot\dfrac{8}{3}=\dfrac{6}{3}\cdot8=8\cdot2=16\)

b: \(\dfrac{5}{x}=\dfrac{4}{9}\)

=>\(x=\dfrac{5\cdot9}{4}=\dfrac{45}{4}\)

c: \(\dfrac{x+3}{-4}=\dfrac{5}{20}\)

=>\(x+3=\dfrac{-4\cdot5}{20}=-1\)

=>x=-1-3=-4

d: \(\dfrac{7}{3+4x}=\dfrac{-2}{9}\)

=>\(4x+3=\dfrac{9\cdot7}{-2}=-\dfrac{63}{2}\)

=>\(4x=-\dfrac{63}{2}-3=-\dfrac{69}{2}\)

=>\(x=-\dfrac{69}{8}\)

f: ĐKXĐ: x<>1

\(\dfrac{3}{x-1}=\dfrac{x-1}{27}\)

=>\(\left(x-1\right)^2=3\cdot27=81\)

=>\(\left[{}\begin{matrix}x-1=9\\x-1=-9\end{matrix}\right.\)

=>\(\left[{}\begin{matrix}x=10\left(nhận\right)\\x=-8\left(nhận\right)\end{matrix}\right.\)

a, \(\dfrac{5}{9}.\dfrac{10}{11}+\dfrac{5}{9}.\dfrac{14}{11}-\dfrac{5}{9}.\dfrac{15}{11}=\dfrac{5}{9}.\left(\dfrac{10}{11}+\dfrac{14}{11}-\dfrac{15}{11}\right)=\dfrac{5}{9}.\dfrac{9}{11}=\dfrac{5}{11}\)

b, \(\dfrac{6}{7}.\dfrac{8}{13}+\dfrac{6}{13}.\dfrac{9}{7}-\dfrac{3}{13}.\dfrac{6}{7}\)\(=\dfrac{6}{7}.\left(\dfrac{8}{13}-\dfrac{3}{13}\right)+\dfrac{6}{13}.\dfrac{9}{7}=\dfrac{6}{7}.\dfrac{5}{13}+\dfrac{54}{91}=\dfrac{30}{91}+\dfrac{54}{91}=\dfrac{84}{91}=\dfrac{12}{13}\)

Bạn ơi, gõ latex cho dễ nhìn nhé!

5.(3x+8)-7.(2x+3)=16

=> 15x+40-14x-21=16

=>(15x-14x)+(40-21)

=> x+19=16

=>x=-3. Vậy x=-3

k cho mik nha

Chúc bạn hok tốt

\(5\left(3x+8\right)-7\left(2x+3\right)=16\)

\(15x+40-14x-21=16\)

\(x+19=16\)

\(x+19-16=0\)

\(x+3=0\)

\(x=-3\)