Bài 2: Phân số bằng nhau

hình chỉ là tham khỏa thôi nhá

a) Ta có: \(\dfrac{x}{3}=\dfrac{7}{y}\)

nên xy=21

b) Ta có: \(\dfrac{x}{y}=\dfrac{1}{6}\)

nên y=6x

c) Ta có: \(\dfrac{x}{7}=\dfrac{y}{-3}\)

nên -3x=7y

a, \(\dfrac{6}{x-3}=\dfrac{9}{2x-7}\)

=> 6(2x-7) = 9(x-3)

=> 12x - 42 = 9x - 27

=> 12x - 9x = -27 + 42

=> 3x = 15 

=> x = 5

Vậy x = 5

b, \(\dfrac{-7}{x+1}=\dfrac{6}{x+27}\)

=> -7(x + 27) = 6(x + 1)

=> -7x - 189 = 6x + 6

=> -7x - 6x = 6 + 189

=> -13x = 195

=> x = -15

Vậy x = -15

a) Ta có: \(\dfrac{6}{x-3}=\dfrac{9}{2x-7}\)

\(\Leftrightarrow6\left(2x-7\right)=9\left(x-3\right)\)

\(\Leftrightarrow12x-42=9x-27\)

\(\Leftrightarrow12x-9x=-27+42\)

\(\Leftrightarrow3x=15\)

hay x=5

Vậy: x=5

b) Ta có: \(\dfrac{-7}{x+1}=\dfrac{6}{x+27}\)

\(\Leftrightarrow6\left(x+1\right)=-7\left(x+27\right)\)

\(\Leftrightarrow6x+6=-7x+189\)

\(\Leftrightarrow6x+7x=189-6\)

\(\Leftrightarrow13x=183\)

hay \(x=\dfrac{183}{13}\)

Vậy: \(x=\dfrac{183}{13}\)

\(\dfrac{25}{30}=\dfrac{2x+3}{6}\)

\(\Rightarrow2x+3=\dfrac{25\cdot6}{30}=5\)

\(\Rightarrow2x=2\)

\(\Rightarrow x=1\)

Ta có: \(\dfrac{25}{30}=\dfrac{2x+3}{6}\)

\(\Leftrightarrow\dfrac{2x+3}{6}=\dfrac{5}{6}\)

\(\Leftrightarrow2x+3=5\)

\(\Leftrightarrow2x=2\)

hay x=1

Vậy: x=1

Ta có : \(\dfrac{a}{b}=\dfrac{c}{d}\)

\(\Leftrightarrow\)\(\dfrac{a}{b}+1=\dfrac{c}{d}+1\)

\(\Leftrightarrow\)\(\dfrac{a+b}{b}=\dfrac{c+d}{d}\)(đpcm)

Đặt a/b=c/d=K

=>a=b.K ; c=d.K

Thay a=b.K ; c=d.K vào biểu thức ta có:

(a+b)/b=(b.K+b)/b=b.(K+1)/b=K+1          (1)

(c+d)/d=(d.K+d)/d=d(K+1)/d=K+1           (2)

Từ (1) và (2)=>Với a/b=c/d thì (a+b)/b=(c+d)/d

        \(-\dfrac{x}{2}=-\dfrac{8}{x}\)

<=> x² = 16

<=> x = 4 hoặc x = -4

Vậy x = 4 hoặc x = -4

\(\frac{2}{-6}=\frac{3}{-9}=\frac{-9}{27}\)

\(\frac{-6}{2}=\frac{-9}{3}=\frac{27}{-9}\)

\(\frac{x}{2}=\frac{3}{y}\)⇔\(xy=2.3=6\).

Ta có bảng:

thanks

Ta có: x.y = 4.(-5)

x.y = (-20)

Vì -20 = (-20).1 = 20.(-1) = (-4).5 = 4.(-5)

Suy ra x = (-20); 20; (-4); 4

a) x/7 = 6/21

<=> x . 21 = 7 . 6

<=> x . 21 = 42

<=> x = 42 : 21

<=> x = 2

b) -5/y = 20/28

<=> y . 20 = -5 . 28

<=> y . 20 = -140

<=> y = -140 : 20

<=> y = -7

Vậy y = -7

a) x/7 = 6/21

<=> x . 21 = 7 . 6

<=> x . 21 = 42

<=> x = 42 : 21

<=> x = 2