Question 1093154
1+2+3...+81=  solve it
1+2+3...+80 --> 80 numbers
Make 40 pairs:
1 + 80 = 81
2 + 79 = 81
etc
40*81 = 3240
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Add 81
--> 3321
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If you prefer remembering rules and formulas:
Sum = n*(n+1)/2 = 81*41
= 3321