Let x & y be differentiable functions of t and let s = sqrt(x^2+y^2) be the distance between the points (x,0) and (0,y) in the xy-plane. How is ds/dt related to dx/dt if y is constant? So I attempted to implicitly take the derivatives of the changing rates. ds/dt= 1/(2sqrt(x^2+y^2)) times 2x dx/dt + 2y Which simplifies to ds/dt= x/(sqrt(x^2+y^2)) dx/dt + y/(sqrt(x^2+y^2)) I'm guessing there is something I'm not understanding because he book shows an answer of ds/dt= x/(sqrt(x^2+y^2)) dx/dt So what y does the y disappear? Or what am I doing incorrectly?