I'm going so deep into the theory of things on my second semester. I'm getting into real analysis and partial derivatives, and it all seems so conceptual. The practicality of integration and differentiation are apparent, but less so in real analysis and partial derivatives, especially when the proofs pertaining to partial differentiation seem to me a computational miracle than anything else. I need good textbooks that touch on the usage of real analysis and partial derivatives (more is welcome, of course). Do you have any suggestions? I want to feel that the proof I'm reading has applications in software companies. That this rule is used in calculating interest at Fortune 500 companies... something like that.