Bài 1:

\(\frac{4}{12}+\frac{4}{20}+\frac{4}{30}+...+\frac{4}{306}\)

\(=4\cdot\left(\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+...+\frac{1}{306}\right)\)

\(=4\cdot\left(\frac{1}{3\cdot4}+\frac{1}{4\cdot5}+\frac{1}{5\cdot6}+...+\frac{1}{17\cdot18}\right)\)

\(=4\cdot\left(\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{17}-\frac{1}{18}\right)\)

\(=4\cdot\left(\frac{1}{3}-\frac{1}{18}\right)\)

\(=4\cdot\left(\frac{6}{18}-\frac{1}{18}\right)\)

\(=4\cdot\frac{5}{18}\)

\(=\frac{10}{9}\)

Bài 2  :

\(\left(3x-4\right)-\left(6x+7\right)=8\)

\(3x-4-6x-7=8\)

\(\left(3x-6x\right)-\left(4+7\right)=8\)

\(-3x-11=8\)

\(-3x=8+11\)

\(-3x=19\)

\(x=19:\left(-3\right)\)

\(x=\frac{-19}{3}\)

Vậy  \(x=\frac{-19}{3}\)

b ) \(\left(\frac{4}{5}x+3\right):\left(-4\right)=\frac{1}{2}\)

\(\frac{4}{5}x+3=\frac{1}{2}\cdot\left(-4\right)\)

\(\frac{4}{5}x+3=-2\)

\(\frac{4}{5}x=\left(-2\right)-3\)

\(\frac{4}{5}x=-5\)

\(x=\left(-5\right):\frac{4}{5}\)

\(x=\left(-5\right)\cdot\frac{4}{5}\)

\(x=-4\)

Vậy   \(x=-4\)

k nha !

\(\frac{4}{12}\)+\(\frac{4}{20}\)+...+\(\frac{4}{306}\)=\(\frac{4}{3.4}\)+\(\frac{4}{4.5}\)+...+\(\frac{4}{17.18}\)=4(\(\frac{1}{3}\)-\(\frac{1}{4}\)+\(\frac{1}{4}\)-\(\frac{1}{5}\)+...+\(\frac{1}{17}\)-\(\frac{1}{18}\))

                                                                                                  =4(\(\frac{1}{3}\)-\(\frac{1}{8}\))=4.\(\frac{5}{24}\)=\(\frac{5}{6}\)

Cảm ơn 2 bạn đã giúp mình nha😜😝🙏🙏🙏🙏

a)3+x=7

x=7-3

x=4

b)-3x+5=-22

-3x=-22-5

-3x=-27

x=-27:(-3)

x=9

d)(-x+3)(2x+4)=0

-x+3=0 hoặc 2x+4=0

x=3 hoặc x=-2

Phần a;b tính dễ dàng nên bạn tự làm nha

c)(-3).|x-1|=-9

=>|x-1|=(-9):(-3)

=>|x-1|=3

=>x-1 =3 hoặc x-1=-3

=>x =3+1 =>x =-3+1

=>x =4 =>x =-2

Vậy \(x\in\left\{4;-2\right\}\)

d)\(\left(-x+3\right).\left(2x+4\right)=0\)

=>-x+3=0 hoặc 2x+4=0

\(\Rightarrow-x=-3\) \(\Rightarrow2x=-4\)

\(\Rightarrow x=3\) \(\Rightarrow x=-4:2\)

\(\Rightarrow x=-2\)

Vậy \(x\in\left\{3;-2\right\}\)

Chúc bn học tốt

I'm... years old. I study at ... secondary school. From Monday to Saturday, i have five classes for each morning. After school, i have many activities. I often play soccer of badminton with my friends after i helping my mom with chores. I often read book in one hour, then i listen to music or watch TV. I sometimes helps my father plant trees behind our house. I find these activities useful.