fatcousin
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1+2+…+n = n(n+1)/2 · double the staircase to form a rectangle · runs locally
staircase
double → rectangle
⊞
n
6
T(n) = 1+2+…+n = n(n+1)/2 = 21 · Gauss age 10: write the series twice (reversed), sum = n×(n+1) = 42, halve it = 21 · visually: two staircases form a 6×7 rectangle
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