a: M là trung điểm của AB
=>\(MA=MB=\dfrac{AB}{2}=\dfrac{36.4}{2}=18,2\left(cm\right)\)
N là trung điểm của BC
=>\(NB=NC=\dfrac{BC}{2}=18,4\left(cm\right)\)
\(S_{MAD}=\dfrac{1}{2}\times MA\times AD=\dfrac{1}{2}\times18,4\times36,8=18,4^2\left(cm^2\right)\)
\(S_{MBN}=\dfrac{1}{2}\times BM\times BN=\dfrac{1}{2}\times18,4\times18,4=\dfrac{18.4^2}{2}\left(cm^2\right)\)
\(S_{NCD}=\dfrac{1}{2}\times NC\times CD=\dfrac{1}{2}\times18,4\times36,8=18,4^2\left(cm^2\right)\)
\(S_{ABCD}=AB\times BC=36,8^2\)
\(S_{MDN}=S_{ABCD}-S_{AMD}-S_{MBN}-S_{NCD}\)
\(=36,8^2-18,4^2-18,4^2-18,4^2:2=507,84\left(cm^2\right)\)
b: \(\dfrac{S_{MDN}}{S_{ABCD}}=\dfrac{507.84}{36,8^2}=\dfrac{3}{8}\)
