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a=3n+1*5n+3n*5n+1+7*3n*5n=

Răspuns :

3x3^nx5^n +3^nx5^nx5 +7x3^nx5^n = 
3^n x5^n(3+5+7 ) =15^n x15  =15^(n+1)
A=3n+1*5n+3n*5n+1+7*3n*5n
A=3n+5n+15n²+1+105n²
A=8n+120n²+1, n∈R