Question #301573

let x1=1 and Xn+1=squrrt(3xn)for all n€N show that (xn) is bounded ,monotone and converges to 3

1
Expert's answer
2022-02-24T15:47:01-0500

Let f(x)=3xf(x)=\sqrt{3x} for all x>0x>0, then we have xn+1=f(xn)x_{n+1}=f(x_n) for all nNn\in\mathbb{N}.

Since f(x)f(x) is a monotonously increasing function, then any recurrent sequence satisfying xn+1=f(xn)x_{n+1}=f(x_n) is monotone. Really:

If x0x1x_0\leq x_1 then f(x0)f(x1)f(x_0)\leq f(x_1), that is, x1x2x_1\leq x_2 and, by induction, xnxn+1x_n\leq x_{n+1}.

If x0x1x_0\geq x_1 then f(x0)f(x1)f(x_0)\geq f(x_1), that is, x1x2x_1\geq x_2 and, by induction, xnxn+1x_n\geq x_{n+1}.

By the condition, x0=1x_0=1, thus, x1=3>1=x0x_1=\sqrt{3}>1=x_0, therefore, this sequence is monotonously increasing. Note that if for any nn, xn<3x_n< 3 then xn+1=f(xn)<f(3)=3x_{n+1}=f(x_n)<f(3)=3, and, by induction, xn<3x_n< 3 for all nNn\in\mathbb{N}. Thus, the given sequence is increasing and bounded from above by 3. Therefore, xnx_n is convergent. Let aRa\in\mathbb{R} is limit of this sequence.

Since a=limn+xna=\lim\limits_{n\to+\infty}x_n and f(x)f(x) is continuous everywhere on (0,+)(0,+\infty), we have

f(a)=limn+f(xn)=limn+xn+1=af(a)=\lim\limits_{n\to+\infty}f(x_n)=\lim\limits_{n\to+\infty}x_{n+1}=a

3a=a\sqrt{3a}=a implies a=0a=0 or a=3a=3. Since xnx_n is increasing, limn+xnx0=1\lim\limits_{n\to+\infty}x_n\geq x_0=1, therefore, a0a\ne 0, and hence, a=3a=3.


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