Question

Square $ABCD$ has area $200$. Point $E$ lies on side $\overline{BC}$. Points $F$ and $G$ are the midpoints of $\overline{AE}$ and $\overline{DE}$, respectively. Given that quadrilateral $BEGF$ has area $34$, what is the area of triangle $GCD$?

Answer 1

1. Consider square ABCD. You know that  

$A_{ABCD}=AD^2=200,$

then

$AB=BC=CD=AD=10\sqrt{2}.$

2. Consider traiangle AED. F is mipoint of AE and G is midpoint of DE, then FG is midline of triangle AED. This means that

$FG=\dfrac{AD}{2}=\dfrac{10\sqrt{2} }{2}=5\sqrt{2}.$

3. Consider trapezoid BFGC. Its area is

$A_{BFGC}=\dfrac{FG+BC}{2}\cdot h,$ where h is the height of trapezoid and is equal to half of AB. Thus,

$A_{BFGC}=\dfrac{FG+BC}{2}\cdot \dfrac{AB}{2}=\dfrac{5\sqrt{2}+10\sqrt{2}}{2}\cdot \dfrac{10\sqrt{2}}{2}=75.$

4.

$A_{BFGC}=A_{BFGE}+A_{EGC},\\A_{EGC}=A_{BFGC}-A_{BFGE}=75-34=41.$

5. Note that angles EGC and CGD are supplementary and

$\sin \angle CGD=\sin \angle EGC.$

Then

$A_{CGD}=\dfrac{1}{2}CG\cdot CD\cdot \sin \angle CGD=\dfrac{1}{2}CG\cdot EG\cdot \sin \angle CGE=A_{ACG}=41.$

Answer: $A_{CGD}=41.$