Răspuns:
Explicație pas cu pas:
6^n+1 = 2^n+1*3^n+1
2^n+2*6^n+1 = 2^n+2*2^n+1*3^n+1 = 2^2n+3*3^n+1
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a = 2^2n+3*3^n+1 - 5*2^2n+1*3^n+2 + 7*2^2n+3*3^n+1
= 2^2n+1*3^n+1*(2^2 - 5*3 + 7*2^2)
= 2^2n+1*3^n+1*(4 - 15 + 28)
= 17*2^2n+1*3^n+1 este divizibil cu 17
