my biggest problem is the word problems. i can't figure out how to write the equation so that i can do the math... Heat is escaping at a constant rate (dQ/dt is constant) through the walls of a long cylindrical pipe. Find the temperature T at a distance r from the axis of the cylinder if the inside wall has a radius r=1 and a temperature T=100, and the outside wall has r=2 and T=0. so i couldn't figure out how to model this. dQ/dt= C doesn't get you very far... maybe cause i'm not sure how to relate the heat to temperature. then i had another problem: A substance evaporates at a rate proportional to the exposed surface. If a spherical mothball of radius 1/2 sm has a radius .4cm after 6 months, how long will it take for the radius to be 1/4cm? i started with like dS/dt= k(SA), where S is the amount of substance and SA was the surface area, (which for a sphere is dependent on r, so this was looking good) but then i got confused about whether i need a negative sign or what else because the substance is lessening. then i wondered if the negative would be just be inherent in the solution, cause it ought to be i thought... i don't know. i'm just getting confused.