1: do, goes => Does, go

2: are => is

3: for => from

k cho mk nha

 1.Do he goes to the birthday party ?            Sửa: Do -> Does ;; Goes -> Go

2. There are 1 cat in my house.                     Sửa : Are -> is

3. This is a frog, It's far for me.                      Sửa : For -> form

b) Ta có: \(a+b-c=0\)

\(\Leftrightarrow a+b=c\)

\(\Leftrightarrow\left(a+b\right)^3=c^3\)

\(\Leftrightarrow a^3+3a^2b+3ab^2+b^3-c^3=0\)

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

\(\Leftrightarrow a^3+b^3-c^3=-3abc\)

=> đpcm

\(=\sqrt{x-2+1+2\sqrt{x-2}}-\sqrt{x-2}\)

\(\sqrt{\left(\sqrt{x-2}+1\right)^2}-\sqrt{x-2}\)

\(\sqrt{x-2}+1-\sqrt{x-2}\)2

=1

Ta có: \(2+2^2+2^3+2^4+...+2^{10}\)

\(=\left(2+2^2+2^3+2^4+2^5\right)+\left(2^6+2^7+2^8+2^9+2^{10}\right)\)

\(=2\left(1+2+2^2+2^3+2^4\right)+2^6\left(1+2+2^2+2^3+2^4\right)\)

\(=2\cdot31+2^6\cdot31\)

\(=31\cdot\left(2+2^6\right)⋮31\)

cửa hàng đó có 0,9 tấn kg gạo đúng ko

Sửa đề : ... tấn gạo tẻ ....

7/10 tấn = 700kg; 1/5 tấn = 200kg

Cửa hàng đó có số gạo là : 700 + 200 = 900 ( kg gạo )

Đ/s : 900kg gạo
 

a) \(a^3+b^3-c^3+3abc\)

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

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

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

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

b) \(\left(x+y+z\right)^3-x^3-y^3-z^3\)

\(=\left(x+y+z-x\right)\left[\left(x+y+z\right)^2+\left(x+y+z\right)x+x^2\right]-\left(y+z\right)\left(y^2-yz+z^2\right)\)

\(=\left(y+z\right)\left[x^2+y^2+z^2+2\left(xy+yz+zx\right)+x^2+xy+zx+x^2-y^2+yz-z^2\right]\)

\(=\left(y+z\right)\left(3x^2+3xy+3yz+3zx\right)\)

\(=3\left(y+z\right)\left[x\left(x+y\right)++z\left(x+y\right)\right]\)

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

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

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

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

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