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Algebra Difficulty 6.4 National olympiad Prove it

Example 11.18 Prove: k=1448sin(x+k)x+k>52\sum_{k=1}^{448} \frac{|\sin (x+k)|}{x+k}>\frac{5}{2}.

Solution

Proof First, we prove the lemma
sin(x1)+sinx+sin(x+1)2sin1|\sin (x-1)|+|\sin x|+|\sin (x+1)| \geqslant 2 \sin 1

Let the function f(x)=sin(x1)+sinx+sin(x+1)f(x)=|\sin (x-1)|+|\sin x|+|\sin (x+1)|. It is easy to see that f(x)f(x) is a periodic function with period π\pi, and f(πx)=f(x)f(\pi-x)=f(x), so we only need to prove that it holds for 0xπ20 \leqslant x \leqslant \frac{\pi}{2}.
(1) When 1xπ21 \leqslant x \leqslant \frac{\pi}{2}, we have
f(x)=sin(x1)+sinx+sin(x1)=sinxcos1cosxsin1+sinx+sinxcos1+cosxsin1=(2cos1+1)sinx(2cosπ3+1)sinx=2sinx2sin1\begin{aligned} f(x)= & \sin (x-1)+\sin x+\sin (x-1)= \\ & \sin x \cos 1-\cos x \sin 1+\sin x+\sin x \cos 1+\cos x \sin 1= \\ & (2 \cos 1+1) \sin x \geqslant\left(2 \cos \frac{\pi}{3}+1\right) \sin x=2 \sin x \geqslant 2 \sin 1 \end{aligned}
(2) For 0x10 \leqslant x \leqslant 1, we have
f(x)=sin(1x)+sinx+sin(1+x)=2sin1cosx+sinxf(x)=\sin (1-x)+\sin x+\sin (1+x)=2 \sin 1 \cos x+\sin x

Let ϕ=arctan(2sin1)\phi=\arctan (2 \sin 1), it is easy to see that $\frac{\pi}{4} \\
\frac{2}{3} \sin 1\left(\frac{1}{1+\frac{\pi}{3}}+\frac{1}{2+\frac{\pi}{3}}+\frac{1}{3+\frac{\pi}{3}}+\sum_{k=4}^{149} \int_{k}^{k+1} \frac{\mathrm{d} x}{k+\frac{\pi}{3}}\right)= \\
\frac{2}{3} \sin 1\left(\frac{1}{1+\frac{\pi}{3}}+\frac{1}{2+\frac{\pi}{3}}+\frac{1}{3+\frac{\pi}{3}}+\int_{4}^{150} \frac{\mathrm{d} x}{k+\frac{\pi}{3}}\right)= \\
\frac{2}{3} \sin 1\left(\frac{1}{1+\frac{\pi}{3}}+\frac{1}{2+\frac{\pi}{3}}+\frac{1}{3+\frac{\pi}{3}}+\ln \left(150+\frac{\pi}{3}\right)-\ln \left(4+\frac{\pi}{3}\right)\right)>
\end{array}
2.5033>522.5033>\frac{5}{2}
Thus, the original inequality is proved.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.