3a^n(1-a)-2[a^n-a^(n+1)]=3a^n-3a^(n+1)-2a^n+2a^(n+1)=a^n-a^(n+1)
(x^2-1)/(X+1)+x^2*(1-1/x)=x-1+x(x-1)=x^2-1