m^2+n^2+2mn
△AOD∽△COB
过O作BC垂线OF,延长FO交AD于M
OF/OM=n/m
∴OF/FM=n/(m+n)
∴S△COB/S△CAB=n/(m+n)
∴S△CAB=mn+n^2=S△CDB
∴S△COD=S△BOA=mn
∴梯形的面积为m^2+n^2+2mn
m^2+n^2+2mn
△AOD∽△COB
过O作BC垂线OF,延长FO交AD于M
OF/OM=n/m
∴OF/FM=n/(m+n)
∴S△COB/S△CAB=n/(m+n)
∴S△CAB=mn+n^2=S△CDB
∴S△COD=S△BOA=mn
∴梯形的面积为m^2+n^2+2mn