Tricks on finding the sum of a sequence from 1 to a selected 2-digit number.

1. Choose a 2-digit number
2. If the number is odd, multiply it by half the number next to it. If the number is even, multiply half of it by the number next to it.

Example 1:

Find the sum of the sequence: 1,2,3,......., 33, 34

Step 1: The two-digit number number is 34

Step 2: The number next to 34 is 35. Multiply 35 by half of 34 which is 17.

           17 x 35 = 595


Therefore, the sum of the sequence 1,2,3,......., 33, 34 is 595.


Example 2:

Find the sum of the sequence: 1,2,3,.......,10, 11

Step 1: The two-digit number number is 11

Step 2: The number next to 11 is 12. Multiply 11 by half of 12 which is 6.

           11 x 6 = 66

Therefore, the sum of the sequence 1,2,3,.......,10,11 is 66.