The probability density function of x, the lifetime of a certain type of electronic device (measured in hours), is given by (a) (b) (c) ab (a + b)2(a + b + 1) ⎧ ⎪⎨ 1 0 x > 1 0 f(x) = ⎪x2 ⎩0 x 10 find p{x > 20}. what is the cumulative distribution function of x? what is the probability that, of 6 such types of devices, at least 3 will function for at least 15 hours? what assumptions are you making?