Why is according to the law of sines $sinA=\frac{a}{2R}$ question
Why is according to the law of sines $sinA=\frac{a}{2R}$

...
i understand the law of sines but I do not understand how this is possible, i need an explanation of where the radius came from 

reminder
R is the radius of a circumcircle
You just need to see the proof of law of sines. See this Or you just take a look at this picture:
The law of sines says that ($d$ - diameter of circumcircle)
$$\frac a {\sin A} =\frac b {\sin B} =\frac c {\sin B} = d $$
$$\Rightarrow \frac {\sin A} {a} = \frac 1 d $$
$$\Leftrightarrow \sin A = \frac a d = \frac a {2r} $$