an=(x^n)/(2*4*.(2n))=x^n/[(2^n)(n!)]
因为lima(n+1)/an=[x^(n+1)/[(2^(n+1))((n+1)!)]]/[x^n/[(2^n)(n!)]]
=lim{x/[2(n+1)]}=0
所以收敛域为R