Well, the AP starts with: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19
Your AP starts with 1 as the first term, then 2nd term is 1+2=3+2=5+2=7.....etc.
The difference from one term to the next is 2. Then, if you add ALL the ten terms listed above, you should get 100. Clear?