1,$(a+b)²=a²+2ab+b²$
2,$(a-b)²=a²-2ab+b²$
3,$a^2-b^2=(a+b)(a-b)$
4,$(a+b)^3=a^3+3a²b+3ab²+b^3$
5,$(a-b)^3=a^3-3a²b+3ab²-b^3$
6,$a^3+b^3=(a+b)(a^2-ab+b^2)$
7,$a^3-b^3=(a-b)(a^2+ab+b^2)$
8,$a^n-b^n=(a-b)(a^{n-1}+a^{n-2}b+....+ab^{n-2}+b^{n-1})$
9,$a^n+b^n=(a+b)(a^{n-1}-a^{n-2}b+....-ab^{n-2}+b^{n-1})$ (n lẻ)
10,$(a_{1}+a_{2}+...+a_{n})^2=a_{1}^2+a_{2}^2+....+a_{n}^2+2(a_{1}a_{2}+a_{1}a_{3}+...+a_{1}a_{n}+a_{2}a_{3}+...+a_{n-1}a_{n}$
11,$(a+b)^n=C_{n}^0a^n+C_{n}a^{n-1}b+...+C_{n}^nb^n$
$C_{n}^k=\frac{n!}{k!(n-k)!}$
12,$a^3+b^3+c^3-3abc=(a+b+c)(a^2+b^2+c^2-ac-bc-ab)$
13,$(a+b+c)^3-a^3-b^3-c^3=3(a+b)(b+c)(c+a)$
