设 u(n)=[(2n-1)!!/(2n!!)] =1/2 * 3/4 * 5/6 * ...... * (2n-1)/(2n) x(n)= 2/3 * 4/5 * 6/7 *......* (2n)/(2n+1) u(n)*u(n) < u(n)*x(n) = 1/(2n+1) 0夹挤准则，=> Limit [u(n),n->∞] =0
