giúp ai alfm vote 5 sao cho CTLHN

a) `8.x+20=76`

`=> 8.x=76-20=56`

`=> x=56:8`

`=> x=7`

b) `10+2.(x-9)=4^5 : 4^3`

`=> 10+2.(x-9)=4^(5-3)`

`=> 10+2.(x-9)=4^2 =16`

`=> 2.(x-9)=16-10=6`

`=> x-9=6:2=3`

`=> x=3+9`

`=> x=12`

c) `x∈ƯC(54; 270)`

Mà:

`54=2.3^3`

`270=2.3^3 .5`

`=> ƯCLN(54; 270)=2.3^3 =54`

`=> x∈ƯC(54; 270)={1; 2; 3; 6; 9; 18; 27; 54}`

Mà `20≤x≤30`

Nên `x=27`

d) `(2^3 x-7).7^10 =7^12`

`=> 8x-7 = 7^12 :7^10 = 7^(12-10) = 7^2 = 49`

`=> 8x=49+7=56`

`=> x=56:8`

`=> x=7`