Euler's representation of Sinc function : ${{\sin \pi x} \over {\pi x}} = \prod_{n=1}^{\infty} \left( 1 - {{x^2} \over {n^2}} \right)$

Question

$\displaystyle {{\sin \pi x} \over {\pi x}} = \prod_{n=1}^{\infty} \left( 1 - {{x^2} \over {n^2}} \right)$

I wanna derive it without complex analysis.

Why (2.8) be below form?