设BC=41X,CA=40X
S^2=p(p-a)(p-b)(p-c)
p=(a+b+c)/2=(9+81X)/2
S^2=(1/16)(81X+9)(81X-9)(9+X)(9-X)
=(81/16)(81X^2-1)(81-X^2)
=-(81^2/16)(((X-3281/81)^2-(41*80/81)^2)
<=(81^2/16)(41*80/81)^2
S<=(81/4)(41*80/81)=820