`a) (3x - 21) . 6^11 = 2. 6^12`
`(3x - 21). 6^11 = 2. (6^11. 6)`
`(3x - 21). 6^11 = (2. 6). 6^11`
`(3x - 21). 6^11 = 12. 6^11`
`=> 3x - 21 = 12`
`=> 3x = 12 + 21`
`=> 3x = 33`
`=> x = 33 : 3`
`=> x = 11`
`b) 5. 2^x = 320`
`2^x = 320 : 5`
`2^x = 64`
`2^x = 2^6`
`=> x = 6`
`c) 4. 3^(x - 1) = 36`
`3^(x - 1) = 36 : 4`
`3^(x - 1) = 9`
`3^(x - 1) = 3^2`
`=> x - 1 = 2`
`=> x = 2 + 1`
`=> x = 3`
`d) 2^x + 5. 2^x = 48`
`2^x. (1 + 5) = 48`
`2^x. 6 = 48`
`2^x = 48 : 6`
`2^x = 8`
`2^x = 2^3`
`=> x = 3`
`e) 3^(x + 1) + 5. 3^x = 72`
`3^x. 3 + 5. 3^x = 72`
`3^x. (3 + 5) = 72`
`3^x. 8 = 72`
`3^x = 72 : 8`
`3^x = 9`
`3^x = 3^2`
`=> x = 2`
`f) 625 : 19^x = 25^2`
`625 : 19^x = 625`
`19^x = 625 : 625`
`19^x = 1`
`19^x = 19^0`
`=> x = 0`
`i) (x - 5)^50 = (x - 5)^18`
- Vì `0^n = 0` `(n ∈ N`*`)` và `1^n = 1` `(n ∈ N)`
`=> x = 0 ; 1`
`k) (x^2 - 7)^2 = 4`
`(x^2 - 7)^2 = 2^2`
`=> x^2 - 7 = 2`
`=> x ^2 = 2 + 7`
`=> x^2 = 9`
`=> x^2 = 3^2`
`=> x = 3`
`l) (x^3 - 5)^2 = 9`
`(x^3 - 5)^2 = 3^2`
`=> x^3 - 5 = 3`
`=> x^3 = 3 + 5`
`=> x ^3 = 8`
`=> x^3 = 2^3`
`=> x = 2`
`m) (x^2 - 6) ^4 = 81`
`(x^2 - 6)^4 = 3^4`
`=> x^2 - 6 = 3`
`=> x^2 = 3 +6`
`=> x^2 = 9`
`=> x^2 = 3^2`
`=> x = 3`
`n) 16 <= 2^x <= 128`
`=> 2^4 <= 2^x <= 2^7`
`=> 4 <= x <= 7`
`=> x = 4 ; 5 ; 6 ; 7`
