Bài 3 : Tìm x , biết.

a) x/3 = 8/12 .

b) x+1/15 = 1/3 .

c) -2/7 + 4/7 × x = 5/7 .

d) 3/4 : x - 5/4 = -1/2 .

Bài 4 : Tìm x , biết.

a) 4/15 : x = 1/2 .

b) 3/5 × x - 1/2 = 1/7 .

c) 2/3 + 3/4 × x = -5/6 .

d) 3/4 × ( x + 2/3 ) = 1 và 1/3 .

Đáp án:

Bài 1: 

a,x=2

b,x=4/15

c,x=7/4

d,x=1

Bài 4:

a,x=8/15

b,x=15/14

c,x=2

d,x=10/9

Giải thích các bước giải:

Bài 3:

a,x/3=8/12

Hay x/3=2/3

⇒x=2

b,x+1/15=1/3

⇒x=1/3-1/15=5/15-1/15=4/15

c,-2/7+4/7.x=5/7

⇒4/7.x=5/7+2/7=7/7=1

⇒x=1:4/7=1.7/4=7.4

d,3/4:x-5/4=-1/2

⇒3/4:x=-1/2+5/4=-2/4+5/4=3/4

x=3/4:3/4=1

Bài 4:

a,4/15:x=1/2

⇒x=4/15:1/2=4/15.2=8/15

b,3/5.x-1/2=1/7

⇒3/5.x=1/7+1/2=2/14+7/14=9/14

⇒x=9/14:3/5=9/14.5/3=15/14

c,2/3+3/4.x=-5/6

⇒3/4.x=-5/6-2/3=-5/6-4/6=-3/2

⇒x=-3/2:-3/4=3/2.4/3=2

d, 3/4.(x+2/3)=1$\frac{1}{3}$ 

⇒3/4.(x+2/3)=$\frac{1.3+1}{3}$ =4/3

⇒x+2/3=4/3:3/4=4/3.4/3=16/9

⇒x=16/9-2/3=16/9-6/9=10/9

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