Tell me the sum of 1 to 10?

55!!1+2+3+4+5+6+7+8+9+10=55

55. and 1 to 100 is 5050~~

tn=n(n+1)/2 10(10+1)/2 10*11/2=55 so the sum is 55

easy... it's 55

65

Pair up the numbers this way: 1 + 10 2 + 9 3 + 8 4 + 7 5 + 6 Notice that the sum of each pair is 11, and there are 5 pairs. And it's easy to do 5 x 11 in your head to get 55. Now try it for the numbers 1 to 20; there are 20 numbers, so 10 pairs, and each pair adds to 21 (1+20, 2+19, etc). So you have 10 x 21 = 210.