Question

In class we are using special sum formulas but don't know how to get the answer for this question

Answer 1

You are referring to:

$\sum\limits_{i=1}^{n}{i} = \frac{n(n+1)}{2}$

You can solve your problem by calculating

$\sum\limits_{i=1}^{525}{i} = \frac{525\cdot526}{2} = 138075$

and then subtracting the first bit:

$\sum\limits_{i=1}^{19}{i} = 190$

So 138075 - 190 = 137885