H € CD BH vuong CD
D'C' // DC cach DC =1/5.DC=80/5=16
D'C' cat BH =H'
ABHD ; ABH'D' la hinh chu nhat
S∆BCH=1/2.HC.AB=1/2.(80-60).40=20.20
S∆BH'C'=1/2.BH'.H'C'=1/2.(40-16).H'C'=12H'C'
Sht (H'C'CH)=1 /2.(H'C'+HC).HH'=8H'C'+8.20
Sht (H'c'ch)+S∆Bh'c'=S∆bch
<=>20H'C'+20.8=20.20
H'C'=20-8=12
S∆BH'C'=12.12
Shcn ABH'D'=60.24
dien tich con lai la
12.12+60.24=12.132=1320+2.132
Ds: 1584(m^2)