A nice problem but it has nothing to do with "Calculus".
I would start by noticing that, on the right, we have "DICI+ AHC= DEEC". Since there is no possible "carry" into the units place, I+ C= C tells us that I= 0. In the middle column, we have DAJ+ IBJ= IGI. So we have J+ J= I= 0. Either J= 0, which, assuming two different letters cannot represent the same digit, is impossible or J= 5 so that J+ J= 10 and the "1" is carried to the tens column. As Dr. Peterson said (Blast him! He keeps getting in just ahead of me! I started before he did but he types faster.) The leading "D" must be "1".