Răspuns:
a=(3²¹ +3²⁰ +3¹⁹)/39
a=3¹⁹(3² +3¹+1)/39
a=3¹⁹·13/39 se reduce 13 si 39
a=3¹⁹/3
a=3¹⁸ =(3⁹)²este pp
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p = 3⁷¹·4⁶²
U(3¹) = 3
U(3²) = 9
U(3³) = 7
U(3⁴) = 1
71 = 17·4+3 => U(3⁷¹) = 7
U(4¹) = 4
U(4²) = 6
62 = 31·2 => U(4⁶²)=6
U(3⁷¹·4⁶²) = U(7·6)=2
ultima cifra este 2
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7ˣ+7ˣ⁺²=2450
7ˣ(1+7²)=2450
50·7ˣ=2450
7ˣ=2450:50=245:5=49
7ˣ=49=>7ˣ=7²=>x=2
