Ta có: \(xy=140\Rightarrow x=\dfrac{140}{y}\)

\(5x-y=15\)

\(\Leftrightarrow5.\dfrac{140}{y}-y=15\)

\(\Leftrightarrow\dfrac{700}{y}-\dfrac{y^2}{y}=15\)

\(\Leftrightarrow\dfrac{700-y^2}{y}=15\)

\(\Rightarrow y=20\)

\(\Rightarrow x=\dfrac{140}{y}=\dfrac{140}{20}=7\)

ta có: 5x - y =15 => y = 5x - 15

=> x.(5x - 15) = 140

=> 5x^2 - 15x -140 = 0

=> 5.(x^2 -3x -28) = 0

=> x^2 - 4x + 7x -28 = 0

=> x(x-4) +7.(x-4) = 0

=> (x+7).(x-4)=0

=> x= -7 hoặc x=4

Nếu x=-7 => y= -50

Nếu x=4 => y=5

vậy hệ phương trình có cặp nghiệm là: (x;y)=(-7;-50) hoặc( x;y) = ( 4;5)

Bài 1:

a) Ta có: \(82-7\left(3x-4\right)=47\)

\(\Leftrightarrow82-21x+28-47=0\)

\(\Leftrightarrow-21x+63=0\)

\(\Leftrightarrow-21x=-63\)

hay x=3(nhận)

Vậy: x=3

b) Ta có: \(97+4\left(5x-7\right)=129\)

\(\Leftrightarrow97+20x-28-129=0\)

\(\Leftrightarrow20x-60=0\)

\(\Leftrightarrow20x=60\)

hay x=3(nhận)

Vậy: x=3

c) Ta có: \(\left(7x-13\right)\cdot27-12=15\)

\(\Leftrightarrow189x-351-12-15=0\)

\(\Leftrightarrow189x-378=0\)

\(\Leftrightarrow189x=378\)

hay x=2(nhận)

Vậy: x=2

d) Ta có: \(\left(2x+3\right)\cdot13+23=140\)

\(\Leftrightarrow26x+39+23-140=0\)

\(\Leftrightarrow26x-78=0\)

\(\Leftrightarrow26x=78\)

hay x=3(nhận)

Vậy: x=3

đ) Ta có: \(52x+8x-5x=70\)

\(\Leftrightarrow55x=70\)

\(\Leftrightarrow x=\frac{70}{55}\)(loại)

Vậy: x∈∅

e) Ta có: \(19x-3x-x=60\)

\(\Leftrightarrow15x=60\)

hay x=4(nhận)

Vậy: x=4

g) Ta có: \(7\left(3x+1\right)-5\left(3x+1\right)=74\)

\(\Leftrightarrow2\left(3x+1\right)=74\)

\(\Leftrightarrow3x+1=37\)

\(\Leftrightarrow3x=36\)

hay x=12(nhận)

Vậy: x=12

h) Ta có: \(5\left(3x-1\right)+7\left(3x-1\right)=96\)

\(\Leftrightarrow12\left(3x-1\right)=96\)

\(\Leftrightarrow3x-1=8\)

\(\Leftrightarrow3x=9\)

hay x=3(nhận)

Vậy: x=3

phép 2 bài 1 đề là 

2^3.3-(1^10+15):4^2

      120 - { 300: [ 40 - (2.562 - 262.5)] + 10}

=    120 - { 300 : [ 40 - (-186)] + 10}

=    120 - {300 : 226 + 10}

=  120 - \(\dfrac{1280}{113}\)

= \(\dfrac{12280}{113}\)

2³.3 - (1¹⁰ + 15): 4²

8.3 - 16:16

= 24 - 1

= 23

Trả lời

25%.x+15/4.x-x:2=140

4/5.x+15/4.x-x.1/2=140

=>x(4/5+15/4-1/2) =140

=>x.9/10                =140

=>x                        =140:9/10

=>x                        =1400/10

Trả lời

Sorry nha, mk còn thiếu bước cuối.

1400/10=140

 <=> \(\frac{1}{4}x+\frac{15}{4}x-\frac{x}{2}=140\)

 <=>\(4x-\frac{x}{2}=140\)

 <=>\(\frac{7}{2}x=140\Rightarrow x=40\)

\(a̸\)   

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

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

           \(\frac{1}{2}.x=\frac{7}{20}\)

            \(\Rightarrow\frac{7}{10}\)

\(b̸\)

\(11,3+2\left[x-\frac{1}{3}\right]=\frac{25}{6}\)

\(2\left[x-\frac{1}{3}\right]=\frac{25}{6}-11,3\)

\(2\left[x-\frac{1}{3}\right]=\frac{-107}{15}\)

\(x-\frac{1}{3}=\frac{-107}{15}:2\)

 \(x-\frac{1}{3}=\frac{-107}{30}\)

\(x=\frac{-107}{30}+\frac{1}{3}\)

\(x=\frac{-97}{30}\)

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

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

\(\implies\frac{1}{2}x=\frac{7}{20}\)

\(\implies x=\frac{7}{20}:\frac{1}{2}\)

\(\implies x=\frac{7}{10}\)

Vậy...

\(b) 11,3+2(x-\frac{1}{3})=\frac{25}{6}\)

\(\implies \frac{113}{10}+2x-2.\frac{1}{3}=\frac{25}{6}\)

\(\implies \frac{113}{10}+2x-\frac{2}{3}=\frac{25}{6}\)

\(\implies \frac{113}{10}+2x=\frac{25}{6}+\frac{2}{3}\)

\(\implies \frac{113}{10}+2x=\frac{29}{6}\)

\(\implies 2x=\frac{29}{6}-\frac{113}{10}\)

\(\implies 2x=\frac{-97}{15}\)

\(\implies x=\frac{-97}{30}\)

Vậy..

\(c)5x-435+2x+140+3x=565\)

\(\implies (5x+2x+3x)+(-435+140)=565\)

\(\implies 10x+(-295)=565\)

\(\implies 10x=565-(-295)\)

\(\implies 10x=860\)

\(\implies x=86\)

Vậy...

~ hok tốt a~

\(5x^2-15x-140=0\)

Ta có \(\Delta=15^2+4.5.140=3025,\sqrt{\Delta}=55\)

\(\Rightarrow\orbr{\begin{cases}x=\frac{15+55}{10}=7\\x=\frac{15-55}{10}=-4\end{cases}}\)

Bài làm

5x² - 15x - 140 = 0

<=> 5x² + 35x - 20x - 140 = 0

<=> 5x( x + 7 ) - 20( x - 7 ) = 0

<=> ( x - 7 )( 5x - 20 ) = 0

<=> x - 7 = 0 hoặc 5x - 20 = 0

<=> x = 7 hoặc x = 4

Vậy S = { 7;4}

Mik nhầm bưosc 3 nha.

5x² - 15x - 140 = 0

<=> 5x² + 35x - 20x - 140 = 0

<=> 5x( x + 7 ) -20( x + 7 ) = 0

<=> ( x + 7 )( 5x - 20 ) = 0

Vậy x = -7 hoặc x = 4

Đáp án A

\(\left(\frac{2}{5}x:10\right)\cdot\left(\frac{3}{5}x:15\right)=4\)

\(\Leftrightarrow\left(\frac{2}{5}x\cdot\frac{1}{10}\right)\cdot\left(\frac{3}{5}x\cdot\frac{1}{15}\right)=4\)

\(\Leftrightarrow\frac{1}{25}x\cdot\frac{1}{25}x=4\)

\(\Leftrightarrow x\left(\frac{1}{25}+\frac{1}{25}\right)=4\)

\(\Leftrightarrow x\cdot\frac{2}{25}=4\)

\(\Leftrightarrow x=50\)

(2/5x : 10) x ( 3/5x : 15) = 4

(2/5x  x 1/10) x ( 1/5x   x 1/15) = 4

1/ 25x    x     1/25x = 4

1/625x² = 4

x² = 4 : 1/625

x² = 2500

x = 50

\(\left(\frac{2}{5}x:10\right)\left(\frac{3}{5}x:15\right)=4\)

=> \(\left(\frac{x}{25}\right).\left(\frac{x}{25}\right)=4\)

=> \(\left(\frac{x}{25}\right)^2=4\)

=> \(\orbr{\begin{cases}\frac{x}{25}=2\\\frac{x}{25}=-2\end{cases}}\Rightarrow\orbr{\begin{cases}x=50\\x=-50\end{cases}}\)

Vậy \(x\in\left\{50;-50\right\}\)