Gauss method for addition

Use the commutatie property of addition to quickly and accurately add the series of numbers below. Remember the method is more important than the answer here.

As the myth goes, young Gauss's elementary school teacher wanted to keep his students busy so he asked the students to add up the numbers 1 to 100. A minute or so later, Gauss had an answer. Can you do the same?

  1. 1 + 2 + 3 + … + 98 + 99 + 100
  2. 5 + 6 + 7 + … + 48 + 49 + 50
  3. -100 + -99 + -98 + … + 13 + 14 + 15
  4. 2 + 4 + 6 + … + 96 + 98 + 100
  5. 5 + 10 + 15 + … + 190 + 195 + 200
  6. 1 + 4 + 7 + … + 52 + 55 + 58
  7. 60 + 67 + 74 + … + 179 + 186 + 193
  8. -5 + -1 + 3 + … + 71 + 75 + 79

Challenge

Describe a general method that works for all the expressions above.