Bài 1:

a) \(=\dfrac{8}{15}\left(\dfrac{7}{13}+\dfrac{6}{13}\right)=\dfrac{8}{15}.1=\dfrac{8}{15}\)

b) \(=\dfrac{3.3-7-2.4}{12}=-\dfrac{6}{12}=-\dfrac{1}{2}\)

Bài 2:

 \(\dfrac{x}{2,7}=-\dfrac{2}{3,6}\Rightarrow x=\dfrac{\left(-2\right).2,7}{3,6}\Rightarrow x=-\dfrac{3}{2}\)

Bài 3:

\(\dfrac{x}{2}=\dfrac{y}{5}=\dfrac{x+y}{2+5}=-\dfrac{21}{7}=-3\)

\(\Rightarrow\left\{{}\begin{matrix}x=\left(-3\right).2=-6\\y=\left(-3\right).5=-10\end{matrix}\right.\)

a\(\left(x-3\right)^2-\left(x+2\right)^2-5\left(\frac{1}{5}-7\right)=-30\)

=>(x-3-x-2)(x-3+x+2)-x+35=-30

=>-5(2x-1)-x+35=-30

=>-10x+5-x+35=-30

=>-11x+40=-30

=>-11x=-70 =>x=70/11

d)\(\left(x+3\right)^2-\left(x+5\right)\left(x-5\right)=2\)

\(=>\left(x+3\right)^2-x^2+25=2\)

\(=>\left(z+3-z\right)\left(z+3+z\right)+25=2\)

\(=>3\left(2z+3\right)+25-2=0\)

\(=>6z+9+23=0\)

\(=>6x+32=0=>6x=-32=>x=-\frac{16}{3}\)

e)\(3\left(x+2\right)^2+\left(2x-1\right)^2-7\left(x+3\right)\left(x-3\right)=36\)

\(=>3\left(x^2+4x+4\right)+\left(4x^2-4x+1\right)-7\left(x^2-9\right)=36\)

\(=>3x^2+12x+12+4x^2-4x+1-7x^2+63\)

\(=>8x+76=36=>8x=36-76=>x=-40\div8=-5\)

g)\(\left(x-1\right)\left(x^2+x+1\right)-x\left(x+2\right)\left(x-2\right)=5\)

\(=>x^3-1-x\left(x^2-4\right)=5=>x^3-1-x^3+4x=5\)

\(=>4x-1=5=>4x=6=>x=\frac{3}{2}\)

a) \(\dfrac{2}{5}+\dfrac{11}{15}=\dfrac{6}{15}+\dfrac{11}{15}=\dfrac{17}{15}\)

b) \(\dfrac{7}{8}-\dfrac{7}{9}=\dfrac{63}{72}-\dfrac{56}{72}=\dfrac{7}{72}\)

c) \(\dfrac{11}{13}\cdot\dfrac{26}{31}=\dfrac{26}{13}\cdot\dfrac{11}{31}=\dfrac{22}{31}\)

d) \(\dfrac{1}{2}:\dfrac{1}{3}\cdot\dfrac{2}{5}=\dfrac{1}{2}\cdot3\cdot\dfrac{2}{5}=\dfrac{3}{5}\)

a ) 2/5 + 11/15 = 6/15 + 11/ 15 = 17/15.

b ) 7/8 -7/9 = 63/72 -56/72 = 7/72.

c ) 11/13 x 26/31 = 22/31

d ) 1/2 : 1/3 x 2/5= 1/2 x 3/1 x 2/5 = 3/5

đúng thì tick mình nhé!!! cảm ơn bạn

B

b

b

\(a)x=\dfrac{1}{4}+\dfrac{5}{13}=\dfrac{33}{52}.\\ b)\dfrac{x}{3}=\dfrac{2}{3}+\dfrac{-1}{7}.\\ \Leftrightarrow\dfrac{x}{3}=\dfrac{11}{21}.\\ \Leftrightarrow\dfrac{7x}{21}=\dfrac{11}{21}.\\ \Rightarrow7x=11.\\ \Leftrightarrow x=\dfrac{11}{7}.\\ c)\dfrac{x}{3}=\dfrac{16}{24}+\dfrac{24}{36}=\dfrac{2}{3}+\dfrac{2}{3}=\dfrac{4}{3}.\\ \Rightarrow x=4.\\ d)\dfrac{x}{15}=\dfrac{1}{5}+\dfrac{2}{3}=\dfrac{13}{15}.\\ \Rightarrow x=13.\)

E, F, G, H, I tí nữa Thầy rảnh Thầy giải giúp nhé!