The temperature T in a metal disc in the x-y plane is given by T=sqrt(x^2+y^2).
Explain why, at any point in the disc, the direction of greatest increase in temperature is given by a vector that points away from the origin.
The direction of greatest in crease is givne by |grad(T)|. The level curves are circles centred at the origin with the equation x^2+y^2= k (k=constant).
From here I am totally unsure what to do...