Proof First, we prove the lemma
∣sin(x−1)∣+∣sinx∣+∣sin(x+1)∣⩾2sin1
Let the function f(x)=∣sin(x−1)∣+∣sinx∣+∣sin(x+1)∣. It is easy to see that f(x) is a periodic function with period π, and f(π−x)=f(x), so we only need to prove that it holds for 0⩽x⩽2π.
(1) When 1⩽x⩽2π, we have
f(x)=sin(x−1)+sinx+sin(x−1)=sinxcos1−cosxsin1+sinx+sinxcos1+cosxsin1=(2cos1+1)sinx⩾(2cos3π+1)sinx=2sinx⩾2sin1
(2) For 0⩽x⩽1, we have
f(x)=sin(1−x)+sinx+sin(1+x)=2sin1cosx+sinx
Let ϕ=arctan(2sin1), 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>25
Thus, the original inequality is proved.