1 + 2 = 3 1 + 2 + 3 = 6 1 + 2 + 3 + 4 = 10 1 + 2 + 3 + 4 + 5 = 15 dst... 1 + 1 = 2 12 + 21 = 33 123 + 321 = 444 1234 + 4321 = 5555 dst...

1. mathematic