设 xn<=3
xn+1=√6+Xn<=√6+3=3即xn有界
xn+1-xn=√6+xn-√6+xn-1=(xn-xn-1)/[√6+Xn+√6+Xn-1]
所以xn+1-xn和xn-xn-1 符号相同
而x2=√6+X1=4,x2-x1<0
所以xn+1-xn<0即xn+1<xn
{xn}是减函数,
所以单调有界数列必有极限;
设极限=a
则limXn+1=lim√6+Xn
a=√6+a
a2=6+a即a2-a-6=0
(a+2)(a-3)=0由a>0可得
a=3即极限=3