Rate of Change

[flakine]

If y=x^3+2x and dx/dt=5, find dy/dt when x=2.

Why can't you that the deriv of y (2x^2+2) and substitute for x?

 

[jacket81]

Well, wouldn't dy/dx=3x^2+2, so then dy/dt=(dy/dx)(dx/dt) = (3x^2+2)5 =

15x^2+10.

So, then substitute in x=2.