1. 1 + 2 + 3 + ... + n = n(n+1)/2
2. n! = n(n-1)(n-2)...(3)(2)(1)
4. a^2 - b^2 = (a + b)(a - b)
5. (a + b)^3 = a^3 + 3a^2b + 3ab^2 + b^3
2. (a - b)^5 = a^5 - 5a^4b + 10a^3b^2 - 10a^2b^3 + 5ab^4 - b^51
3. (a + b)^6 = a^6 + 6a^5b + 15a^4b^2 + 20a^3b^3 + 15a^2b^4 + 6ab^5 + b^61
4. (a - b)^6 = a^6 - 6a^5b + 15a^4b^2 - 20a^3b^3 + 15a^2b^4 - 6ab^5 + b^61