(6)a1=2a(n+1) = (1/2)(an + 1/an )an >0{ an} 递减lim(n-> ∞ ) an 存在letlim(n-> ∞ ) an = La(n+1) = (1/2)(an + 1/an )L = (1/2)(L + 1/L )2L^2 -L-1 =0(2L +1)(L-1)=0L =1
