30 x 60 = 1800
x = 10 S = 49/4 * 100 - 225 = 1225 < 1800 => sì
x = 20 S = 49/4 * 400 - 225 = 4675 > 1800 => no
x = 30 S > 4675, inutile calcolarlo => no
1)
30*60 = 1800 cm^2
2)
(a+b)*(a-b) = a^2-b^2
49x^2/4 -225
3)
A(10) = 49*10^2/4-225 = 1.000 cm^2 < 1800 : Sì
A(20) = 49*20^2/4-225 = 4.675 cm^2 > 1800 : No
A(30) = 49*30^2/4-225 = 10.800 cm^2 > 1800 : No