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Autumn Tournament

Algebra Difficulty 5.5 AIME, harder Prove it Bulgaria

Let x0,x1,…x_0, x_1, \dots be a sequence of real numbers such that x0=1x_0 = 1 and xn+1=sin⁡(xn)+π2−1x_{n+1} = \sin(x_n) + \frac{\pi}{2} - 1 for all n≥0n \ge 0. Show that the sequence converges and find its limit.
(Kristyan Vasilev)

Solution

We will first prove that the sequence is strictly increasing. Note that for each nn we have xn≤π2x_n \le \frac{\pi}{2}, since sin⁡x≤1\sin x \le 1 for each x∈Rx \in \mathbb{R}. Besides we have that the function f(x)=sin⁡x−xf(x) = \sin x - x is decreasing for x∈Rx \in \mathbb{R}, because f′(x)=cos⁡x−1≤0f'(x) = \cos x - 1 \le 0 for each x∈Rx \in \mathbb{R}.

Therefore, for x∈(−∞,π2)x \in (-\infty, \frac{\pi}{2}) we have f(x)≥sin⁡π2−π2f(x) \ge \sin \frac{\pi}{2} - \frac{\pi}{2}. So we get that sin⁡xn≥xn+1−π2\sin x_n \ge x_n + 1 - \frac{\pi}{2}, which is equivalent to xn+1≥xnx_{n+1} \ge x_n. Therefore, the series (xn)n=1∞(x_n)_{n=1}^\infty is increasing and since it is bounded it follows that it is convergent. If ll is its limit, then for ll it is true that l=sin⁡l+π2−1l = \sin l + \frac{\pi}{2} - 1, i.e. f(l)=f(π2)f(l) = f(\frac{\pi}{2}). The function ff is decreasing, which means that l=π2l = \frac{\pi}{2}.

π2\boxed{\frac{\pi}{2}}

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