Additions curieuse

1 + 2 = 3

4 + 5 + 6 = 7 + 8

9 + 10 + 11 + 12 = 13 + 14 + 15

16 + 17 + 18 + 19 + 20 = 21 + 22 + 23 + 24

25 + 26 + 27+ 28 + 29 + 30 = 31 + 32 + 33 + 34 + 35