$#nobody$

`7) 3^(x + 2) + 3^(x - 1) <= 28`

`<=> 3^x . 3^2 + 3^x . 3^(- 1) <= 28`

`<=> 19/3 . 3^x <= 28`

`<=> 3^x <= 84/19`

`<=> x <= log_3 (84/19)`

Vậy...

`8) 2^(2x - 1) + 2^(2x - 2) + 2^(2x - 3) >= 448`

`<=> 2^(2x) . 2^(- 1) + 2^(2x) . 2^(- 2) + 2^(2x) . 2^(- 3) >= 448`

`<=> 2^(2x) . 7/8 >= 448`

`<=> 2^(2x) >= 512`

`<=> 4^x >= 512`

`<=> x >= log_4 512 = 9/2`

Vậy ...

`9) (0,4)^x - (2.5)^(x + 1) > 1,5`

`<=> (2/5)^x - (5/2)^(x + 1) > 3/2`

`<=> (1/(5/2))^x - (5/2)^x . 5/2 > 3/2`

`<=> 1/((5/2)^x) - (5/2)^x . 5/2 > 3/2`

Đặt `(5/2)^x = t(t > 0)`.Ta được PT:

`1/t - t . 5/2 > 3/2`

`<=> 2/t - 5t > 3`

`<=> (2 - 5t^2)/t > 3`

`<=> 2 - 5t^2 > 3t(t > 0)`

`<=> - 5t^2 - 3t + 2 > 0`

`<=> 5t^2 + 3t - 2 < 0`

`<=> (t + 1)(5t - 2) < 0`

`+) {(t + 1 > 0),(5t - 2 < 0):} <=> {(t > - 1),(t < 2/5):} <=> - 1 < t < 2/5`

`+) {(t + 1 < 0),(5t - 2 > 0):} <=> {(t <- 1),(t > 2/5):}(L)`

Mà `t > 0 = > 0 < t < 2/5`

`=> 0 < (5/2)^x < 2/5`

`<=> {((5/2)^x > 0(lđ)),((5/2)^x < 2/5):} <=> x < - 1`

Vậy ..