原式=(a-1)^(2n-1)+2(1-a)^2n+(a-1)^(2n+1)
=(a-1)^(2n-1)+2(a-1)^2n+(a-1)^(2n+1)
=(a-1)^(2n-1)[1+2(a-1)+(a-1)^2]
=(a-1)^(2n-1)(1+a-1)^2
=(a-1)^(2n-1)a^2