1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10.100 = how much, how to calculate with the formula

1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10.100 = how much, how to calculate with the formula

(1+100)*100/2=5050