It is given that triangle $BOC$ makes an angle of $\theta$ with the triangle $ABC$.
I understood the above thing in this way: the normal to the triangle $BOC$ makes an angle of $\theta$ with the normal of the triangle $ABC$. But the problem came when the book wrote $$ area (BOC) = area (ABC) \cos \theta$$ Why the area has to be equal to the projection of area of $ABC$ on triangle $BOC$?