14:

a: Xét ΔHNM vuông tại H và ΔMNP vuông tại M có

góc N chung

=>ΔHNM đồng dạng với ΔMNP

b: NP=căn 3^2+4^2=5cm

MH=3*4/5=2,4cm

NH=3^2/5=1,8cm

13:

a: 3x+5=x-5

=>2x=-10

=>x=-5

b: (x-2)(2x+5)=0

=>x-2=0 hoặc 2x+5=0

=>x=2 hoặc x=-5/2

c: =>2(5x-2)=3(3x+1)

=>10x-4=9x+3

=>x=7

d: =>(3x+6-x+1)/(x+2)(x-1)=17-3x/(x+2)(x-1)

=>2x+7=17-3x

=>5x=10

=>x=2

a: \(VP=a^3+b^3+c^3-3bac\)

\(=\left(a+b\right)^3+c^3-3ab\left(a+b\right)-3abc\)

\(=\left(a+b+c\right)\left[\left(a+b\right)^2-c\left(a+b\right)+c^2\right]-3ab\left(a+b+c\right)\)

\(=\left(a+b+c\right)\left(a^2+2ab+b^2-ac-bc+c^2-3ab\right)\)

\(=\left(a+b+c\right)\left(a^2+b^2+c^2-ab-bc-ac\right)=VT\)

b: \(VT=\left(3a+2b-1\right)\left(a+5\right)-2b\left(a-2\right)\)

\(=3a^2+15a+2ab+10b-a-5-2ab+4b\)

\(=3a^2+14a+14b-5\)

\(VP=\left(3a+5\right)\left(a+3\right)+2\left(7b-10\right)\)

\(=3a^2+9a+5a+15+14b-20\)

\(=3a^2+14a+14b-5\)

=>VT=VP

c: \(VT=a\left(b-x\right)+x\left(a+b\right)\)

\(=ab-ax+ax+bx\)

\(=ab+bx=b\left(a+x\right)=VP\)

d: \(VT=a\left(b-c\right)-b\left(a+c\right)+c\left(a-b\right)\)

\(=ab-ac-ab-bc+ca-cb\)

\(=-2bc\)

=VP

Theo đề, ta có:

\(PTK_{X_2O_3}=NTK_X.2+16.3=102\left(đvC\right)\)

\(\Leftrightarrow NTK_X=27\left(đvC\right)\)

Vậy X là nguyên tố nhôm (Al)

Vậy CTHH của A là: Al₂O₃

X₂O₃=102

<=>2X+48=102

<=>2X=54

<=>X=27

=>X:Al

=>CTHH của A:Al₂O₃

9. A
10. D
5. A
6. B
7. D
8. B
 

9 A

10 D

:)

1.B dynamite
2.B disappear
3.B refreshment
4.C disappointed
5.C division
6.C disappointment
7.B congratulate
8.D inedible
9.C congratulation
10.A oceanic

--thodagbun--

1 A

3 C

4 C

5 A

6 B

10 A

1 If we continue polluting the air, the environment will be affected seriously

2 If we don't stop damaging birds' homes, there will be fewer birds left

3 If we continue polluting the water, there will be more disease in people and animals

4 If we keep polluting the land, the soil will not be safe enough for us to grow food in

5 If we use public transportation when we need, there will be less air pollution

12 Daniel signs up for a clean-up campaign so that he can join in it

13 People in my neighborhood clean the park up so that their children will have a nice place to play

14 Almost all the students in my school plant trees so that they can have much fresher air

o: x^4+x^3+x^2-1

=x^3(x+1)+(x-1)(x+1)

=(x+1)(x^3+x-1)

q: \(=\left(x^3-y^3\right)+xy\left(x-y\right)\)

=(x-y)(x^2+xy+y^2)+xy(x-y)

=(x-y)(x^2+2xy+y^2)

=(x-y)(x+y)^2

s: =(2xy)^2-(x^2+y^2-1)^2

=(2xy-x^2-y^2+1)(2xy+x^2+y^2-1)

=[1-(x^2-2xy+y^2]+[(x+y)^2-1]

=(1-x+y)(1+x-y)(x+y-1)(x+y+1)

u: =(x^2-y^2)-4(x+y)

=(x+y)(x-y)-4(x+y)

=(x+y)(x-y-4)

x: =(x^3-y^3)-(3x-3y)

=(x-y)(x^2+xy+y^2)-3(x-y)

=(x-y)(x^2+xy+y^2-3)

z: =3(x-y)+(x^2-2xy+y^2)

=3(x-y)+(x-y)^2

=(x-y)(x-y+3)

o) \(x^4+x^3+x^2-1\)

\(=\left(x^4+x^3\right)+\left(x^2-1\right)\)

\(=x^3\left(x+1\right)+\left(x+1\right)\left(x-1\right)\)

\(=\left(x+1\right)\left(x^3+x-1\right)\)

q) \(x^3+x^2y-xy^2-y^3\)

\(=\left(x^3+x^2y\right)-\left(xy^2+y^3\right)\)

\(=x^2\left(x+y\right)-y^2\left(x+y\right)\)

\(=\left(x+y\right)\left(x^2-y^2\right)\)

\(=\left(x+y\right)^2\left(x-y\right)\)

s) \(4x^2y^2-\left(x^2+y^2-1\right)^2\)

\(=\left(2xy\right)^2-\left(x^2+y^2-1\right)^2\)

\(=\left(2xy-x^2-y^2+1\right)\left(2xy+x^2+y^2-1\right)\)

\(=-\left(x^2-2xy+y^2-1\right)\left(x^2+2xy+y^2-1\right)\)

\(=-\left(x-y-1\right)\left(x-y+1\right)\left(x+y+1\right)\left(x+y-1\right)\)

u) \(x^2-y^2-4x-4y\)

\(=\left(x^2-y^2\right)-\left(4x+4y\right)\)

\(=\left(x+y\right)\left(x-y\right)-4\left(x+y\right)\)

\(=\left(x+y\right)\left(x-y-4\right)\)

x) \(x^3-y^3-3x+3y\)

\(=\left(x^3-y^3\right)-\left(3x-3y\right)\)

\(=\left(x-y\right)\left(x^2+xy+y^2\right)-3\left(x-y\right)\)

\(=\left(x-y\right)\left(x^2+xy+y^2-3\right)\)

z) \(3x-3y+x^2-2xy+y^2\)

\(=\left(3x-3y\right)+\left(x^2-2xy+y^2\right)\)

\(=3\left(x-y\right)+\left(x-y\right)^2\)

\(=\left(x-y\right)\left(3+x-y\right)\)