n =  20 

tick nha !

\(=\dfrac{2-1}{1x2}+\dfrac{3-2}{2x3}+\dfrac{4-3}{3x4}+...+\dfrac{10-9}{9x10}=\)

\(=1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{9}-\dfrac{1}{10}=\)

\(=1-\dfrac{1}{10}=\dfrac{9}{10}\)

6-(y+15)+17

=6-y-15+17

=6-15+17-y

=8-y

vậy kết quả là 8-y

k cho mik zới

ố, ghê nhoa

Câu 2: 

1:

a: a>b

nên -2a<-2b

=>-2a+1<-2b+1

b: a>b

nên 5a>5b

=>5a+1>5b+1>5b-1

2: \(A=x^2+2x+1+4=\left(x+1\right)^2+4>=4\)

Dấu '=' xảy ra khi x=-1

Câu 3:

1) \(\left|x+7\right|=1-2x\) (*)

-ĐK: \(1-2x\ge0\Leftrightarrow-2x\ge-1\Leftrightarrow x\le\dfrac{1}{2}\)

(*) \(\Leftrightarrow x+7=1-2x\) hay \(x+7=2x-1\)

\(\Leftrightarrow x+2x=1-7\) hay \(x-2x=-1-7\)

\(\Leftrightarrow3x=-6\) hay \(-x=-8\)

\(\Leftrightarrow x=-2\) (nhận) hay \(x=8\) (loại)

-Vậy \(S=\left\{-2\right\}\)

2) \(M=a^3+b^3=\left(a+b\right)\left(a^2-ab+b^2\right)=\left(2x+5-x+1\right)\left[\left(2x+5\right)^2-\left(2x+5\right)\left(-x+1\right)+\left(-x+1\right)^2\right]=\left(x+6\right)\left[\left(2x+5\right)^2-\left(2x+5\right)\left(-x+1\right)+\left(-x+1\right)^2\right]>0\)

\(\Rightarrow x+6>0\) và \(\left[\left(2x+5\right)^2-\left(2x+5\right)\left(-x+1\right)+\left(-x+1\right)^2\right]>0\) hoặc \(x+6< 0\) và \(\left[\left(2x+5\right)^2-\left(2x+5\right)\left(-x+1\right)+\left(-x+1\right)^2\right]< 0\)

-Mà \(\left[\left(2x+5\right)^2-\left(2x+5\right)\left(-x+1\right)+\left(-x+1\right)^2\right]\) luôn lớn hơn 0 (bạn tự c/m)

\(\Rightarrow x>-6\)

\(a,=\dfrac{20}{60}+\dfrac{12}{60}-\dfrac{15}{60}\\ =\dfrac{17}{60}\\ b,=\dfrac{7}{8}-\left(\dfrac{5}{20}+\dfrac{8}{20}\right)\\ =\dfrac{7}{8}-\dfrac{13}{20}\\ =\dfrac{35}{40}-\dfrac{26}{40}=\dfrac{9}{40}\\ c,=\dfrac{5}{11}-\dfrac{3}{55}=\dfrac{25}{55}-\dfrac{3}{55}=\dfrac{22}{55}=\dfrac{2}{5}\)

a: =20/60+12/60-15/60=17/60

b: =7/8-1/4-2/5=5/8-2/5=25/40-16/40=9/40

c: =5/11-3/5+6/11=1-3/5=2/5

a:17/60

b:9/40

c:2/5

11 on

12 emigrated

13 to leave

14 up

15 ....

16 outside

17 will make

18 transmitted

19 exhibition

20 to

11:on.

12:emigrated.

13:to leave.

14:up.

15...

16:outside.

17:will make

18:transmitted.

Chán quá, ít ng trl 

A nhé bn

Tính ra đi

là câu C