∵lim(n->∞)│[(2n+1)x^(2n)/2^(n+1)]/[(2n-1)x^(2n-2)/2^n]│
=lim(n->∞){[(2n+1)x²]/[2(2n-1)]}
=lim(n->∞){[(2+1/n)x²]/[2(1-1/n)]}
=x²/2
令x²/2│x│