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)#.