>>807
IS=∬[△A1A2A3]aTBx/|a|dS
dS=△A1A2A3/△OA1A2dx1dx2
=|a|A3dx1dx2
=a3/|a|dx1dx2
IS=∬[△OA1A2]aTBxa3/|a|^2dx1dx2
=you can do it