[math problem 3 ^ 2-1 = 8 = 8 * 1; 5 ^ 2-3 ^ 2 = 16 = 8 * 2; 7 ^ 2-5 ^ 2 = 24 = 8 * 3; 9 ^ 2-7 ^ 2 = 32 = 8 * 4

[math problem 3 ^ 2-1 = 8 = 8 * 1; 5 ^ 2-3 ^ 2 = 16 = 8 * 2; 7 ^ 2-5 ^ 2 = 24 = 8 * 3; 9 ^ 2-7 ^ 2 = 32 = 8 * 4

a^2-b^2=(a+b)(a-b)
2011^2-2009^2=(2011+2009)(2011-2009)=8040=8*1005