a,b,c=laturile triunghiului
a/5=b/6=c/9=k
a=5k
b=6k
c=9k
a+b+c=5k+6k+9k-=20k
20k=40
k=40:20=2
a=5x2=10
b=6x2=12
c=9x2=18
Aria=√p(p-a)(p-b)(p-c) (formula lui Heron)
p=a+b+c/2 (semiperimetrul)
p=10+12+18/2=40/2=20
A=√20(20-10)(20-12)(20-18)
A=√20x10x8x2=√3200=40√2
