Define y = sqrt(arcsin(x^2-1)) and make y^2=arcsin(x^2-1)->x^2-1=sin(y^2). Now following with f(x,y)=x^2-sin(y^2) = 1
We know that (dy)/(dx) = -f_x/(f_y) = (2x)/(2y cos(y^2)). Now, substituting for y = sqrt(arcsin(x^2-1)) we have (dy)/(dx) = x/(sqrt(arcsin(x^2-1))cos(arcsin(x^2-1))
Simplifying (dy)/(dx) =x/(sqrt(arcsin(x^2-1))sqrt[1 - (1 - x^2)^2])