function dx=cdp4ffun(w,x)
cao=0.037; 
v0=50.0;
t1=((1.0-x)^0.5)*(2.5-2.0*x);
t2=(1.0-0.2*x)^(1.5);
t3=2.5*(cao^0.5)/v0;
dx=t3*t1/t2;