>>302
∂r/∂x=(∂/∂x)√(x^2+y^2)=x/√(x^2+y^2)=c
∂θ/∂x=(∂/∂x)arctan(y/x)=-y/(x^2+y^2)=-s/r

(∂/∂r)(Zr*c)=Zrr*c+Zr*(∂c/∂r)=Zrr*c+Zr*0