The temperature T in a metal disc in inversely proportional to the distance from the center of the disc, which we take to be the origin. The temperature at the point (1,2) is 10 centigrade.

(a) show that at any point the direction of greatest increase in temperature is given by a vectore that points toward the origin.

(b) find the rate of change of T at (1,2) in the direction toward the point (2,1)

I have no idea about (a), and with (b), isnt the rate of change undefined, because rate of change of distance = 0 (distance of both points to the origin are the same = sqrt (5)) and thus (rate of change of T)/(rate of change of D) is undefined??? soooooo confused!! Please help!!!