1. The problem statement, all variables and given/known data i want to derive a formula for an ellipse sector. ellipse is not rotated and its center is in the origin. its semimajor and semiminor axis are a and b, respectively, and angle of the sector begins with t1 and ends with t2. it's just a simple surface integral in polar coordinates, i did that already and got the result. but then i found this: http://mathforum.org/library/drmath/view/53635.html" i also have noticed that at t1 = 0 and t2 = 2pi, the area calculated from that formula does not match A = pi*a*b. there is a correction for the answer given by doctor sam just below the next post, but i have no clue where did that dTheta come from, i can't see a reason why a normal integration wouldn't give correct results. what did i miss? for calculating the area of the whole ellipse, we used the following transformation at lessons: x = a r cos(fi) y = b r sin(fi) J (jacobi determinant) = abr this thing works only for 0 < fi < 2*pi. why? why none of our professors mentioned any of these problems?