Well Ive taken calculus 1-3 all up to differential equations and its been quite easy. Though I forgot most of the stuff already. Im going to be taking upper division math but I feel that my math background is still very weak because all I ever did was computations.I never understood why It works. I mean I tried to prove it but I never got it. I know that I have gaps. Anyone recommend any books or what should I do to close these gaps?