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Đáp án + Giải thích các bước giải:

`(1) x xx2/3 +1/2=5/6`

`x xx2/3=5/6-1/2`

`x xx2/3=5/6-3/6`

`x xx2/3=1/3`

`x=1/3:2/3`

`x=1/2`

Vậy `x=1/2`

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`(2)x + 1/7 xx 1/8= 111/112`

`x+1/56=111/112`

`x=111/112-1/56`

`x=111/112-2/112`

`x=109/112`

Vậy `x=109/112`

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`(3)16/21-x xx2/3=4/7`

`x xx2/3=16/21-4/7`

`x xx2/3=16/21-12/21`

`x xx2/3=4/21`

`x=4/21:2/3`

`x=2/7`

Vậy `x=2/7`

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`(4) x:2+x:3 +x:6=100`

`x xx1/2+ x xx1/3 +x xx1/6=100`

`x xx (1/2+1/3+1/6)=100`

`x xx 1=100`

`x=100:1`

`x=100`

Vậy `x=100`

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`(5)x : 1/7 + x:1/21+x:1/3:x:6=20`

`x xx7 + x xx21 + x xx3 : x :6 =20`

`->` Sai đề.

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`(6) (x-21 xx13) :11 =39`

`(x-273)=39xx11`

`x-273=429`

`x=429+273`

`x=702`

Vậy `x=702`

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`(7) (x-21)xx13 :11=39`

`(x-21)xx13=39xx11`

`(x-21)xx13=429`

`x-21=429:13`

`x-21=33`

`x=33+21`

`x=54`

Vậy `x=54`

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`(8)(x-5)xx(1995xx1996+1996xx1997)=1234xx5678xx(630-315xx2):1996`

`(x-5)xx(1995xx1996+1996xx1997)=1234xx5678xx(630-630):1996`

`(x-5)xx(1995xx1996+1996xx1997)=0`

`x-5=0:(1995xx1996+1996xx1997)`

`x-5=0`

`x=0+5`

`x=5`

Vậy `x=5`