a, b, c-laturile triunghiului
P =a+b+c
a+b+c=40
{a, b, c} dp{5,6,9}
a/5=b/6=c/9=k
a=5k
b=6k
c=9k
5k+6k+9k=40
20k=40
k=40:20
k=2
a=5×2=10 cm
b=6×2=12 cm
c=9×2=18 cm
p=(a+b+c) /2= (10+12+18)/2=40/2=20
A =√(20(20-10)(20-12)(20-18)) = √(20×10×8×2)= √3200=40√2 cm²
