Răspuns:
2.
a=3^n•(1+3^2)
a=3^n•10 sau 3^(n-1)•3•10 sau 3^(n-1)•30 divizibil cu 30.
5.
A(x)=[(2x+1)^2 - (2x+2)^2] - [(2x+3)^2 - (2x+4)^2]
A(x)=[(2x+1+2x+2)(2x+1-2x-2)][(2x+3+2x+4)(2x+3-2x-4)]
A(x)=[-(4x+3)][-(4x+7)]
A(x)=(-4x-3)(-4x-7)
A(x)=16x^2 + 28x + 12x + 21
A(x)=16x^2 + 40x + 21
A(x)=(4x+3)(4x+7) pătrat perfect
ex. x=1 : A(x)=7•28=2^2 • 7^2
