function yp = Fp(y,x)

% remember, this is supposed to return the derivative 
%    d/dx F(y(x),x) where  dy/dx = F(y(x),x)
% i.e. it returns
%    F_y(y(x),x) dy/dx + F_x(y(x),x)= F_y(y(x),x) F(y(x),x) + F_x(y(x),x)

% Remember, you have to compute F_x and F_y by hand and put them in
% for each different problem!
F_x = -2*x/(1+x^2)^2;
F_y = -4*y;
yp = F_x + F_y*F(y,x);