Toate cele trei sunt progresii aritmetice de ratie r=2
a) (11-1) : 2 + 1 = 6 termeni S = 6(1+11):2 = 3x12 = 36 = 6^2
b) (19-1) : 2 + 1= 10 termeni S= 10(1+19):2 = 100 = 10^2
c) (99-1) : 2 + 1= 50 termeni S = 50(1+99):2 = 50x50=50^2
Deci toate simele sunt patrate perfecte.
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