aire (FPM') = aire (OPM') - aire (OFM')

aire (FPM') = \displaystyle \pi a^2 \frac{E}{2\pi} -\frac{1}{2} || \vect{OF} \wedge \vect{OM'} ||

aire (FPM') = \displaystyle \pi a^2 \frac{E}{2\pi} -\frac{1}{2} a^2 e \sin E

aire (FPM') = \displaystyle \frac{1}{2} a^2 (E - e \sin E)