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Aratatu ca numarul a=(3^{21}+3^{20}+3^{19}):39 easte patrat perfect

Răspuns :

a=(3²¹+3²⁰+3¹⁹):39
a=3¹⁹·(3²+3+1):39
a=3¹⁹·13:39
a=3¹⁸·3·13:39
a=3¹⁸·39:39
a=3¹⁸·1
a=3¹⁸ =(3⁹)² este patrat perfect