An=Sn-S(n-1)=[(Sn)^0.5+(S(n-1))^0.5]×[(Sn)^0.5-(S(n-1))^0.5]2An=[(Sn)^0.5+(S(n-1))^0.5]两式相除[(Sn)^0.5-(S(n-1))^0.5]=0.5{(Sn)^0.5}是等差数列
