See complete Problem 180 at:
www.gogeometry.com/problem/p180_circle_triangle_quadrilateral_area.htm
Circles Tangent Externally, Common External Tangents, Areas. Level: High School, SAT Prep, College geometry
Post your solutions or ideas in the comments.
1.
ReplyDeletefor 2 fixed touching circle, CD is unique, so is EF
reflect CD to C'D' by AB, C'D' touch the circle, C'=E and D'=F, and obvious that S_1=S_2
2.
mACD+mCDB=180°, AC//BD, [BCD]=[BAD], S=S_1+S_2 is obvious