is raining 

is taking - hopes - will pass 

are visiting - visit 

did not write - phoned 

did not visit - caught

21 is raining

22 is taking/hopes/will pas

25 are visiting

26 visited

27 didn't write

28 phoned

29 didn't visit

30 caught

1 C

2 A

3 C

4 D

5 B

II

1 C

2 A

3 B

4 C

5 D

1 B

2 A

3 D

4 C

5 C

6 A

7 B

8 D

2 A

3 C

4 B

5 A

6 C

7 B

8 D

9 B

a) Áp dụng định lý pytago có 

AB²+AC²=BC²

thay số 

=>BC=10cm

nhanh zị

lx

lỗi rùi bn

Bài 4: 

a: Xét tứ giác OBAC có 

\(\widehat{OBA}+\widehat{OCA}=180^0\)

Do đó: OBAC là tứ giác nội tiếp

hay O,B,A,C cùng thuộc 1 đường tròn

Bài 5:

\(\sqrt{x+2021}-y^3=\sqrt{y+2021}-x^3\\ \Leftrightarrow\left(\sqrt{x+2021}-\sqrt{y+2021}\right)+\left(x^3-y^3\right)=0\\ \Leftrightarrow\dfrac{x-y}{\sqrt{x+2021}+\sqrt{y+2021}}+\left(x-y\right)\left(x^2+xy+y^2\right)=0\\ \Leftrightarrow\left(x-y\right)\left(\dfrac{1}{\sqrt{x+2021}+\sqrt{y+2021}}+x^2+xy+y^2\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x-y=0\\\dfrac{1}{\sqrt{x+2021}+\sqrt{y+2021}}+x^2+xy+y^2=0\left(1\right)\end{matrix}\right.\)

Dễ thấy \(\left(1\right)>0\) với mọi x,y

Do đó \(x-y=0\) hay \(x=y\)

\(\Leftrightarrow M=x^2+2x^2-2x^2+2x+2022=x^2+2x+1+2021\\ \Leftrightarrow M=\left(x+1\right)^2+2021\ge2021\)

Dấu \("="\Leftrightarrow x=y=-1\)