This is not a specific HW/CW question, just a gap I have and want to fill. I came from a school in which calculus was only introduced in the last year so I learnt only the basics. Now, I see more and more stuff like taking an expression, say A=B+C and simply making it to a dA=dB+dC. The problem I have is actually understanding why can we simply cancel out the 'with respect to' part ? That is in dA(x)/dx, the dx part. I think the main question is: When can we treat dA(x)/dx as a fraction rather then an 'operator' (could not come up with a better for d/dx). Thanks!