let x1=1 and Xn+1=squrrt(3xn)for all n€N show that (xn) is bounded ,monotone and converges to 3
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Expert's answer
2022-02-24T15:47:01-0500
Let f(x)=3x for all x>0, then we have xn+1=f(xn) for all n∈N.
Since f(x) is a monotonously increasing function, then any recurrent sequence satisfying xn+1=f(xn) is monotone. Really:
If x0≤x1 then f(x0)≤f(x1), that is, x1≤x2 and, by induction, xn≤xn+1.
If x0≥x1 then f(x0)≥f(x1), that is, x1≥x2 and, by induction, xn≥xn+1.
By the condition, x0=1, thus, x1=3>1=x0, therefore, this sequence is monotonously increasing. Note that if for any n, xn<3 then xn+1=f(xn)<f(3)=3, and, by induction, xn<3 for all n∈N. Thus, the given sequence is increasing and bounded from above by 3. Therefore, xn is convergent. Let a∈R is limit of this sequence.
Since a=n→+∞limxn and f(x) is continuous everywhere on (0,+∞), we have
f(a)=n→+∞limf(xn)=n→+∞limxn+1=a
3a=a implies a=0 or a=3. Since xn is increasing, n→+∞limxn≥x0=1, therefore, a=0, and hence, a=3.
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