S=S(ABCD)+S(BEFG)-S(AEF)-S(ADC)
=a^2+b^2-a^2/2-(a+b)*b/2
=(a^2+b^2)/2-ab/2
当a=5厘米,b=3厘米时
s=(25+9)/2-1/2*5*3
=17-7.5
=9.5