Find the sum of the finite arithmetic sequence.
1. 2 + 4 + 6 + 8 + 10 + 12 + 14 + 16 + 18 + 20
2. 1 + 4 + 7 + 10 + 13 + 16 + 19
3. −1 + (−3) + (−5) + (−7) + (−9)
4. −5 + (−3) + (−1) + 1 + 3 + 5
5. Sum of the first 100 positive odd integers