`|x-2| >=0`

`<=> |x-2|+4/7 >= 4/7`

`<=> A>=4/7`

`=> A_(min)=4/7 <=> x=2`

a. `A=x^2-5x+7`

`=x^2-2.x. 5/2 + (5/2)^2 +3/4`

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

`=> A_(min) =3/4 <=> x-5/2 =0<=>x=5/2`

b) `B=2x^2-8x+15`

`=[(\sqrt2x)^2 -2.\sqrt2 x . 2\sqrt2 +(2\sqrt2)^2] +7`

`=(\sqrt2x-2\sqrt2)^2+7`

`=> B_(min)=7 <=> x=2`.

`a=m^2+m+1=m^2+2.m. 1/2 + (1/2)^2 + 3/4= (m+1/2)^2 + 3/4 >0 forall m`

`=> a>0 =>` Hàm số luôn đồng biến trên `RR`.

ĐK: ` x \ne \pm 2`

`(x+1)/(x+2)-x/(x-2)=(5x-2)/(4-x^2)`

`<=>(x+1)/(x+2)-x/(x-2)+(5x-2)/(x^2-4)=0`

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

`<=> x^2-x-2-x^2-2x+5x-2=0`

`<=>2x-4=0`

`<=>x=2` (L)

Vậy PTVN.

1) `-3\sqrt13=-3\sqrt13`

`-9=-3\sqrt9`

`\sqrt13>\sqrt9`

`=> -3\sqrt13 < -3\sqrt9`

`=> -3\sqrt13 < 9`.

2) `\sqrt15 < \sqrt16`

`<=> \sqrt15-1 < \sqrt16-1`

`<=> \sqrt15-1 < 3 < \sqrt10`

`=> \sqrt15-1 <\sqrt10`

3) `5=4+1=\sqrt16+1`

`\sqrt8+1=\sqrt8+1`

`=> 5>\sqrt8+1`

ĐK: `x>=0`

`6+\sqrtx-x=0`

`6+3\sqrtx-2\sqrtx-x=0`

`<=>3(2+\sqrtx)-\sqrtx(2+\sqrtx)=0`

`<=>(\sqrtx+2)(3-\sqrtx)=0`

`<=> [(\sqrtx+2=0),(3-\sqrtx=0):}`

`<=> [(\sqrtx=-2 (L)),(\sqrtx=3):}`

`<=>x=9`

Vậy `S={9}`.

`(y+3)^3-(3-y)^2-54y`

`=y^3+9y^2+27y+27-9+6y-y^2-54y`

`=y^3+8y^2-21y+18`

- Làm hết câu dễ rồi, câu khó mời cao nhân. :D

Children of all ages dress up in costumes and travel from house to house and say “trick or treat!" on Halloween night.

It's freezing outside and you've got a cold. Wear that shirt I bought for you to keep your neck warm.

He gave his girlfriend chocolate on Valentine day.

You look beautiful with this yellow dress. The colour matches your blond hair too.

Santa Clauses give children a lot of presents on Christmas Eve.

Vietnamese Independence Day is on September 2nd.

2. I have
3. you must
4. Should I
5. You don't need to
6. I be able
7. I managed
8. I must
9. I may
10. Had I better
11. You must
12. You must