a) x + 15 = 36 - 2x

x + 15 = 36 - (x + x )

15 =36 - ( x + x) - x

15 = 36 - x - x - x

15 = 36 - 3x

3x = 36 - 15

3x = 21

x = 21 : 3

=> x = 7

b) (x - 7) - (2x +5) = -14

x - 7 -( 2x + 5) = -14

x - (2x + 5) = -14 + 7 = -7

x - 2x - 5 = -7

x - 2x = -7 + 5 = -2

x - x + x = 2

x = 2 (-x + x cũng bằng chính nó)

=> x = 2

c) (x - 12) - 15 = (-7 + 20) - (18+x)

(x - 12) - 15 = 13 - (18 + x)

(x - 12) - 15 = 13 - 18 - x

(x - 12) - 15 = -5 - x

15 = (x - 12 ) - (-5 - x)

15 = x - 12 + 5 + x

15 = x + (-12) + 5 + x

15 = 2x + [(-12) + 5]

15 = 2x + -7

2x = -7 + 15

2x = 8

x = 8 : 2

=> x = 4

..................

\(a)\dfrac{1}{3}x+\dfrac{2}{5}\left(x+1\right)=0\)

\(\Leftrightarrow\dfrac{1}{3}x+\dfrac{2}{5}x+\dfrac{2}{5}=0\)

\(\Leftrightarrow x\left(\dfrac{5}{15}+\dfrac{6}{15}\right)=\dfrac{-2}{5}\)

\(\Leftrightarrow x.\dfrac{11}{15}=\dfrac{-2}{5}\)

\(\Leftrightarrow x=\dfrac{-2}{5}.\dfrac{15}{11}\)

\(\Leftrightarrow x=\dfrac{-6}{11}\)

\(xy+12=x+y\)

\(xy-x-y=12\)

\(x\left(y-1\right)-y-1=12-1\)

\(x\left(y-1\right)-\left(y-1\right)=11\)

\(\left(y-1\right)\left(x-1\right)=11\)

Vì \(x,y\in Z\Leftrightarrow y-1;x-1\inƯ\left(11\right)\)

\(\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}y-1=1\\x-1=11\end{matrix}\right.\\\left\{{}\begin{matrix}y-1=-1\\x-1=-11\end{matrix}\right.\\\left\{{}\begin{matrix}y-1=11\\x-1=1\end{matrix}\right.\\\left\{{}\begin{matrix}y-1=-11\\x-1=-1\end{matrix}\right.\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}y=2\\x=12\end{matrix}\right.\\\left\{{}\begin{matrix}y=0\\x=-10\end{matrix}\right.\\\left\{{}\begin{matrix}y=2\\x=12\end{matrix}\right.\\\left\{{}\begin{matrix}y=-10\\x=0\end{matrix}\right.\end{matrix}\right.\)

Vậy ..

-8 chia hết cho x và 12 chia hết cho x

-8\(⋮\)x và 12 \(⋮\)x

=>x\(\in\)ƯC(-8,12)={\(\pm\)1;\(\pm\)2;\(\pm\)4}

Chúc bn học tốt

1:

a)10/20-15/20+16/20=-5/20+16/20

=11/20.

b)2/3+8/3:8/5=2/3+5/3

=7/3.

2:a)Ta có:

2x=1/4=3/4

2x=4/4=1

x=1:2

x=0,5

b)x:(2/12-1/12)=-3/8.

x:1/12=-3/8.

x=-3/8x1/12.

x=-1/32.

a ) \(5\left(x^2\right)+7x+2\)

\(\Leftrightarrow5x^2+7x+2=0\)

\(\Leftrightarrow5x^2+5x+2x+2=0\)

\(\Leftrightarrow\left(5x+2\right)\left(x+1\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}x=-\dfrac{2}{5}\\x=-1\end{matrix}\right.\)

Vậy .............

b ) \(\dfrac{x+1}{17}+\dfrac{x+2}{16}=\dfrac{x+3}{15}+\dfrac{x+4}{14}\)

\(\Leftrightarrow\dfrac{x+1}{17}+1+\dfrac{x+2}{16}+1=\dfrac{x+3}{15}+1+\dfrac{x+4}{14}+1\)

\(\Leftrightarrow\dfrac{x+18}{17}+\dfrac{x+18}{16}=\dfrac{x+18}{15}+\dfrac{x+18}{14}\)

\(\Leftrightarrow\dfrac{x+18}{17}+\dfrac{x+18}{16}-\dfrac{x+18}{15}-\dfrac{x+18}{14}=0\)

\(\Leftrightarrow\left(x+18\right)\left(\dfrac{1}{17}+\dfrac{1}{16}-\dfrac{1}{15}-\dfrac{1}{14}\right)=0\)

Vì \(\left(\dfrac{1}{17}+\dfrac{1}{16}-\dfrac{1}{15}-\dfrac{1}{14}\right)\ne0\)

Ta có : \(x+18=0\Leftrightarrow x=-18\)

Vậy ......

c ) \(\dfrac{x-1}{x-3}=\dfrac{x-4}{x-7}\)

\(\Leftrightarrow\left(x-1\right)\left(x-7\right)=\left(x-3\right)\left(x-4\right)\)

\(\Leftrightarrow x^2-7x-x+7=x^2-4x-3x+12\)

\(\Leftrightarrow-x=5\)

\(\Leftrightarrow x=-5\)

Vậy ..

cảm ơn nhiều nha