Let $S = 2010 + 2011 + \cdots + 4018$. Compute the residue of $S$, modulo 2009.

Notice that

2010 ≡ 1 mod 2009

2011 ≡ 2 mod 2009

2012 ≡ 3 mod 2009

...

4017 ≡ 2008 mod 2009

4018 ≡ 0 mod 2009

So really, S is just the sum of the first 2008 positive integers:

where we invoke the formula

and so S ≡ 0 mod 2009.

b

step-by-step explanation:

a difference is the result of subtraction. while each of these expressions has subtraction in it, a difference will be the result when subtraction is the last operation applied in the order of operations for the expression. this means subtraction must be outside of the parenthesis. only b has this.