>>335
(1) c = a
(2)
F(x) = 1 - exp(-ax)
(3)
P(1<x<2) = F(2) - F(1) = exp(-a) - exp(-2a)
(4)
E[X] = ∫_{x=0 to ∞} a x exp(-ax) dx = 1/a
E[X^2] = 2/(a^2)

Var(X) = E[X^2] - E[X]^2 = 1/(a^2)