Now just plug in x/2=u and you get: (d/dx[x/2])/sqrt(1-(x/2)^2). If we power down we can determine that d/dx[x/2]=1/2. Plug this in and we will get (1/2)/sqrt(1-(x/2)^2). If we simply we get 1/(2sqrt(1-x^2/4)) and this is the derivative of arcsin(x/2).