PLEASE HELP!! Directions: Find the sum for each of the following arithmetic series below. Use the summation formula to find the sums.

1. Find the sum of the first 21 terms in a series where a = 20 and t21= 400.

2. Find the sum of the first 100 terms in a series where a = 0 and t100= 99.

3. Find the sum of the first 50 terms in a series where a = 4 and t50= 196.