设C=90°
则a^2+b^2=c^2
a+b+c=l
l=a+b+√(a^2+b^2)≥2√(ab)+√(2ab)=(2+√2)√ab
√ab≤l/(2+√2)
S=1/2ab=1/2(√ab)^2≤1/2(l/(2+√2))^2=l^2(3-2√2)/4