If x=at4, y=bt3, then dydx=?
3b4at
This is called implicit differentiation or indirect differentiation. Here both the terms given are differentiated independently w.r.t a third variable and then combined together. Differentiating x and y w.r.t 't'
dxdt=4at3, dydt=3bt2
Diving dydt by dxdt, dydx=dydt×dtdx=3bt24at3=3b4at