1/2*3/4...*2n
当n趋近于无穷时
1/2*3/4...*2n-1/2n<2/3*4/5....2n/(2n+1) (1/2*3/4...*2n-1/2n)^2<1/(2n+1) 1/2*3/4...*2n-1/2n<根号下1/(2n+1)->0, 故极限为0