I'm just trying to refresh my calculus a bit, I'm stuck on a question and I'd love some insight.
A square measures 0.9cm on each side when drawn with a pencil. When traced over with a marker, it measures 0.95cm on each side. Use the differential of area, $dA$, to estimate the increase in area of the square.
What I would do is this:
$$s_p = 0.9cm$$ $$s_m = 0.95cm$$ $$ds = s_m- s_p = 0.05cm$$
$$dA = s_m * ds = .95cm * .05cm = 0.475cm^2$$