6. let $(a) = T'(a) / T(a) for a > 0 (the digamma function). Show that
7. Suppose that a regular light bulb, a long-life light bulb, and an extra-long-life light bulb being tested. The lifetime X1 of the regular bulb has an exponential distribution with mean 0, the lifetime X2 of the long-life bulb has an exponential distribution with mean 2©.