Brain Teasers & Mental Math
Quick sum: what is 1 + 2 + 3 + … + 100?
Model answer
5,050.
Gauss pairing: pair the ends — 1+100 = 101, 2+99 = 101, 3+98 = 101 … There are 50 such pairs, so 50 × 101 = 5,050.
General formula: sum of 1 to n = n(n…
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