Question: Problems with computing double integral

Hi. I tried to solve some excersices but with this one I've got a little problem. I was asked to calculate the area of a circlesegment with radius R and midpointcorner 2*alpha. I have chosen the straigt side of the circlesegment to be parallel with the y axis.

I get these formulas in polarco├Ârdinates.

oppp:=Doubleint(integrand(pool),r=R*cos(alpha)/cos(theta)..R,theta=-alpha..alpha);
alpha R
/ /
| |
oppp := | | | r | dr dtheta
| |
/ /
-alpha R cos(alpha)
------------
cos(theta)

I guess I've used the right formule but Maple gives me some problems getting a clear solution. I've tried to tell Maple with the commando assume that R is constant but I can not get a clear solution. Maybe I've typed something wrong. Or maybe I have made a mistake getting to this formula.

Sorry for my Enlgish and thank you in advance.
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