Finding the limit using definite integration $$y=\lim_{n\to \infty}\left(\sin π/2n \times \sin 2π/2n \times... \sin(n-1)π/2n\right)$$ Find y.
 I took log on both the sides and I got 
$$\ln y= \int_0^1 {\ln \sin πx/2\, dx}$$ but I'm stuck here.

If we wish to evaluate the limit of interest by use of Riemann sums, then we can proceed as follows.  Let $y_n$ be the product defined by
$$y_n=\prod_{k=1}^{n-1}\sin\left(\frac{k\pi}{2n}\right) \tag 1$$
Taking the logarithm of both sides of $(1)$ reveals 
$$\log y_n=\sum_{k=1}^{n}\log\left(\sin\left(\frac{k\pi}{2n}\right)\right) \tag  2$$
where we used the fact that $\sin \pi/2=1$, thereby permitting our extending the limit on the sum to $n$.  Note that we can write the Riemann sum of the integral 
$$\begin{align}
\int_0^{\pi/2}\log\left(\sin x\right)\,dx&=\lim_{n\to \infty}\,\frac{\pi/2}{n}\,\sum_{k=1}^{n}\log\left(\sin\left(\frac{k\pi}{2n}\right)\right)\\\\
&=-\frac12 \pi \log 2 \tag 3
\end{align}$$
Comparing the $(2)$ and $(3)$ we see that 
$$\lim_{n\to \infty}\log y_n =-\infty$$
which implies therefore that 
$$\bbox[5px,border:2px solid #C0A000]{\lim_{n\to \infty}y_n=0}$$
