`5x(4x^2-2x+1)-2x(10x^2-5x-2)`

`= 20x^3-10x^2+5x - (20x^3-10x^2-4x)`

`=9x`

Thay `x=15` có: `9.15=135`.

a. `A>=0`.

`(\sqrt(3x^2-12x+12)-x+2)/(x-2)`

`=(\sqrt(3(x^2-4x+4))-(x-2))/(x-2)`

`=(\sqrt(3(x-2)^2)) -(x-2))/(x-2)`

`=(\sqrt3. (x-2) - (x-2))/(x-2)`

`=( (\sqrt3-1) (x-2))/(x-2)`

`=\sqrt3-1`

`=>` C.

Sửa: 

ĐK: \(\dfrac{-x^2-1}{x}\ge0\Leftrightarrow-\dfrac{\left(x^2+1\right)}{x}\ge0\Leftrightarrow x< 0\)

Vì (\(-\left(x^2+1\right)< 0\forall x\)).

ĐK: `(-x^2-1)/x >=0 <=> -(x^2+1)/x >=0 <=> x<=0` (Vì `-(x^2+1) <=0`)

I can fly.

You must do your homework.

She has to wear the uniform.

He may succeed in doing that.

Thay `x=2, y=-3` có: `4.2^3 . (-3)^2=288`

- Bạn bổ sung cả hình ảnh nhớ. :D

`A=-x^2+6x+8=-(x^2-6x-8)`

`=-(x^2-2.x.3 +3^2 -17)`

`=-(x-3)^2+17`

Vì: `(x-3)^2 >= 0 forall x`

`=> -(x-3)^2 <=0 `

`<=>-(x-3)^2+17 <=17`

`=> A_(max) = 17 <=> x-3=0<=>x=3`