P4 - Differentiation? Are they finding the area of the shadowed region on P3?
I don't think so since Hina shows on a different coordinate system on the right page in P4, whereas in P3 the coordinate system is on the left page :P
Actually, the coordinate systems on both P3 and P4 are probably the same; Hina turned the book around so that Sato could see it :P
Well I think basically the area can be evaluated by
Integral(0 to right-intersection-x, (top-line-height - parabola-height) * dx) - Integral(0 to lowerright-intersection-x, (bottom-line-height - parabola-height) * dx)
. But then again, what are we doing picking on such matters in a literally girls' love comic? :P