a: góc DAC=90-40=50 độ

b: góc ADB=90 độ

c: góc DAB=90-80=10 độ

=>góc BAE=10+50=60 độ

góc AED=180-60=120 độ

Trả lời:

Bài 1:

a, \(9x^2-4=\left(3x\right)^2-2^2=\left(3x-2\right)\left(3x+2\right)\)

b, \(x^3+27=x^3+3^3=\left(x+3\right)\left(x^2-3x+9\right)\)

c, \(8-y^3=2^3-y^3=\left(2-y\right)\left(4+2y+y^2\right)\)

d, \(x^4-81=\left(x^2\right)^2-9^2=\left(x^2-9\right)\left(x^2+9\right)\)\(=\left(x^2-3^2\right)\left(x^2+9\right)=\left(x-3\right)\left(x+3\right)\left(x^2+9\right)\)

e, \(64x^3-1=\left(4x\right)^3-1^3=\left(4x-1\right)\left(16x^2+4x+1\right)\)

f, \(x^6+8y^3=\left(x^2\right)^3+\left(2y\right)^3=\left(x^2+2y\right)\left(x^4-2x^2y+4y^2\right)\)

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 \("="\Leftrightarrow x=y=-1\)

1. I don't want to eat anything, I'm not hungry.
2. Thuy and her friends are skipping rope in the school yard now.
3. It's very kind for you to make friend with me
4. I'm going to introduce you to natural wonders in the world.
5. Our children are interested in reading books about history and geography.

76. We can choose friends but we can not choose relatives.
77. we live in a very peaceful neighborhood.
78. my sister is living in a modern apartment in the city.
79. can you tell me the way to the railway station.
80. let us go out for a walk.

Dài thế