BÀI TẬP VỀ TỈ LỆ THỨC , TÍNH CHẤT DÃY TỈ SỐ BẰNG NHAU :

BÀI 1 : Tìm x biết :

a) x/25 = 3/5

b) x/-16 = 15/20

c) x-2/3 = 4/5

d) x+3/-4 = 5/20

e) 2x + 3/7 = 4x - 1/15

f) x - 1/3 = x + 3/5

g) x/2 = 8/x

h) x-2/-27 = -3/x - 2

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Bài `1:`

`a) x/25 = 3/5`

`=> 5x=3.25`

`=> 5x=75`

`=>x=75:5`

`=>x=15`

Vậy `x=15`

`b) x/-16 = 15/20`

`=> x/-16= 3/4`

`=> x/16=(-12)/16`

`=> x=-12`

Vậy `x=-12`

`c) (x-2)/3=4/5`

`=> 5(x-2)=4.3`

`=>5x-10=12`

`=>5x=22`

`=>x=22/5`

Vậy `x=22/5`

`d) (x+3)/-4 = 5/20`

`=> (x+3)/4=(-1)/4`

`=>x+3=-1`

`=>x=-4`

Vậy `x=-4`

`e) (2x+3)/7 = (4x-1)/15`

`=> 15(2x+3)=7(4x-1)`

`=>30x+45=28x-7`

`=> 30x-28x=-7-45`

`=>2x=-52`

`=>x=-26`

Vậy `x=-26`

`f) (x-1)/3=(x+3)/5`

`=>5(x-1)=3(x+3)`

`=>5x-5=3x+9`

`=>5x-3x=9+5`

`=>2x=14`

`=>x=7`

Vậy `x=7`

`g) x/2 = 8/x \ (x ne 0)`

`=> x^2=2.8`

`=> x^2=16`

`=>x^2=(+-4)^2`

`=>x=+-4 \ (tm)`

Vậy `x in {4;-4}`

`h) (x-2)/-27 = (-3)/(x-2) \ (x ne 2)`

`=> (x-2)^2=(-27).(-3)`

`=> (x-2)^2=81`

`=>(x-2)^2=(+-9)^2`

`=>` \(\left[ \begin{array}{l}x-2=9\\x-2=-9\end{array} \right.\) 

`=>` \(\left[ \begin{array}{l}x=11\\x=-7\end{array} \right.\) (tm)

Vậy `x in {11;-7}`

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`*** a/b=c/d=>ad=bc`