f(x) = TAN(α)·x - 5·x^2/(v·COS(α))^2
a)
f(x) = 0
TAN(α)·x - 5·x^2/(v·COS(α))^2 = 0 -- > x = v^2/5·SIN(α)·COS(α) ∨ x = 0
b)
x(α) = v^2/5·SIN(α)·COS(α)
x'(α) = v^2/5·(2·COS(α)^2 - 1) = 0 --> α = pi/4 = 45°
c)
Sx = v^2/10·SIN(α)·COS(α)
h = f(Sx) = TAN(α)·(v^2/10·SIN(α)·COS(α)) - 5·(v^2/10·SIN(α)·COS(α))^2/(v·COS(α))^2
h = v^2/20·SIN(α)^2