Calculus works for linear equations of the 1st order because it is simple geometry, and that cannot be faulted; one does not need calculus for this. Now take a second order equation, the simple function: y = x^2. From rudimentaries, detay y / deltax = 2x + delta x. That is the EXACT equation for the slope. One can make delta x as small as one wants, however the slope changes. Bringing delta x to infinity, and delta y follows, is the manner in which, all calculations are consistent, but not right. Infinity, as is well known, does not exist, for it is a mathematical trick for some aspects of mathematics to exist. Calculus is a manner in which, to normalise calculations throughout the mathematical and engineering field. Comments please.