P = 1/2 * (a+b) * H
a = h + 2
b = h + 4
1/2 * (h + 2 + h + 4) * h = 40 /*2
(2h + 6) * h = 80
2h² + 6h - 80 = 0
Δ = 6² - 4 * 2 * (-80) = 36 + 640 = 676
√Δ = 26
h1 = - 6 + 26 / 4 = 5
h2 = - 6 - 26 / 4 < 0, bok nie może być ujemny!
a = 7 cm
b = 9 cm
h = 5 cm
r² = h² + (b - a)²
r² = 5² + 2²
r² = 29
r = √29 cm
l = 7 + 9 + 5 + √29 = 21 + √29 [cm]