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#camon

`12)`

`a) 3^( x - 1 ) - 3 = 78`

`=> 3^( x - 1 ) = 78 + 3`

`=> 3^( x - 1 ) = 81`

`=> 3^( x - 1 ) = 3^4`

`=> x - 1 = 4`

`=> x = 4 + 1`

`=> x = 5`

Vậy `x = 5`

`b) 4^( x - 1 ) + 3 = 67`

`=> 4^( x - 1 ) = 67 - 3`

`=> 4^( x - 1 ) = 64`

`=> 4^( x - 1 ) = 4^3`

`=> x - 1 = 3`

`=> x = 3 + 1`

`=> x = 4`

Vậy `x = 4`

`c) 3^( x - 4 ) - 63 = 18`

`=> 3^( x - 4 ) = 18 + 63`

`=> 3^( x - 4 ) = 81`

`=> 3^( x - 4 ) = 3^4`

`=> x - 4 = 4`

`=> x = 4 + 4`

`=> x = 8`

Vậy `x = 8`

`d) 4^( x - 3 ) + 15 = 79`

`=> 4^( x - 3 ) = 79 - 15`

`=> 4^( x - 3 ) = 64`

`=> 4^( x - 3 ) = 4^3`

`=> x - 3 = 3`

`=> x = 3 + 3`

`=> x = 6`

Vậy `x = 6`

`13)`

`a) 10 + 2x = 4^5 : 4^3`

`=> 10 + 2x = 4^2`

`=> 2x = 16 - 10`

`=> 2x = 6`

`=> x = 6 : 2`

`=> x = 3`

Vậy `x = 3`

`b) 20 + 5x = 5^5 : 5^3`

`=> 20 + 5x = 5^2`

`=> 5x = 25 - 20`

`=> 5x = 5`

`=> x = 5 : 5`

`=> x = 1`

`c) 4x - 20 = 2^5 : 2^2`

`=> 4x - 20 = 2^3`

`=> 4x = 8 + 20`

`=> 4x = 28`

`=> x = 28 : 4`

`=> x = 7`

Vậy `x = 7`

`d) 15 + 2x = 3^80 : 3^77`

`=> 15 + 2x = 3^3`

`=> 2x = 27 - 15`

`=> 2x = 12`

`=> x = 12 : 2`

`=> x = 6`

Vậy `x = 6`