[Q2] Let X be a continuous random variable with the cumulative distribution function as follows. 0 0.2.x x < 0 0 ≤ x ≤4 F(x) = 0.04x0.64 4≤x≤9 9≤x (a) Write out the probability density function of X. (b) P(2<X<5) = ? (c) Find out the mean and variance of X.