Đáp án:
`81cm^2`
Giải thích các bước giải:
` AM=MD, AM+MD=AD`
`=> AM=MD=1/2 AD = 1/2` x `12 = 6cm`
`DN=NC; DN+NC=DC`
`=> DN=NC=1/2 DC=1/2` x `18 = 9cm`
`S_{ABM} = 1/2` x `AM` x` AB = 1/2` x `6` x `18 = 54 (cm^2)`
`S_{MDN}=1/2` x `MD` x `DN = 1/2` x `6` x `9 = 27 (cm^2)`
`S_{BCN} = 1/2` x `BC` x `CN = 1/2` x `12` x `9 = 54 (cm^2)`
`S_[ABCD}=12`x`18 =216 (cm^2)`
`S_{BMN}=S_{ABCD}-S_{ABM}-S_{MDN} - S_{BCN}`
`=216 - 54 - 27 - 54`
`=81 cm^2`