$#Dream$

`a)121-(118-x)=217`

`121-118+x=217`

`3+x=217`

`x=217-3`

`x=214`

Vậy `x=214`

`b)7x-x`=`5^21`:`5^19`+`3.2^2`-`7`

`7x-x`=`5^2`+`3.4-7`

`7x-x`=`25+12-7`

`7x-x`=`37-7`

`7x-x`=`30`

`7x-1x`=30`

`(7-1)x=30`

`6x=30`

`x=30:6`

`x=5`

Vậy `x=5`

`c)(2x-4)(3-x)=0`

`=>2x-4=0` hoặc `3-x=0`

`+)TH1 : 2x-4=0`

`2x=4+0`

`2x=4`

`x=4:2`

`x=2

`+)TH2:3-x=0`

`x=0-3`

`x=-3`

Vậy `x=2` hoặc `x=-3`

`d)(3x`-`2^4`).`7^3`=`2.7^4`

`(3x`-`2^4`).`7^3`=`4802`

`(3x-16).343=4802`

`3x-16=4802:343`

`3x-16=14`

`3x=16+14`

`3x=30`

`x=30:3`

`x=10`

Vậy `x=10`

Giải thích các bước giải:

 a) `121 - ( 118 - x ) = 217`
`118  - x = 121 - 217`
`118 - x = -96`
`x = 118 - ( -96 )`
`x = 118 + 96`
`x = 214`
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b) `7x - x = 5^21 : 5^19 + 3.2^2 - 7`
`6x = 5^2 + 3 . 4 - 7`
`6x = 25 + 12 - 7`
`6x = 30`
`x = 30 : 6`
`x = 5`
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c) `( 2x - 4 ) (3 - x) = 0`
Trường hợp 1 :
`2x - 4 = 0`
`2x = 0 + 4`
`2x = 4`
`x = 4 : 2`
`x = 2`

Trường hợp 2 :
`3 - x = 0`
`-x = 0 - 3`
`-x = -3`
`x = 3`
Vậy `x = 2 ; x = 3`
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d) `( 3x - 2^4 ) . 7^3 = 2.7^4`
`(3x - 16 ) . 343 = 2. 2401`
`(3x - 16 ) . 343 = 4802`
`3x - 16 = 4802 : 343`
`3x - 16 = 14`

`3x = 14 + 16`
`3x = 30`
`x = 30 : 3`
`x = 10`