课程名称︰机率导论
课程性质︰数学系大二必修
课程教师︰陈宏
开课学院：理学院
开课系所︰数学系
考试日期（年月日）︰2015/03/31
考试时限（分钟）：60
试题 :
1. (20 points) The probability that n electrons are emitted during a certian
interval from a photoelectrically emissive surface under incident light is
n
P(n) = Cp ,
where n can be 0, 1, 2, ... and p is a positive number less than 1. Calculate
the constant C (it will depend on p). (Hint: you do not have to know anything
about Physics to solve this problem.)
2. (24 points) A random variable X has a cumulative distribution function given
by
╭ 0 x ＜ 0
│ 3 4
F (x) = ╯ 4x - 3x 0 ≦ x ＜ 1
X │
╰ 1 x ≧ 1
Find P(X = 1/2), P(X = 5), and P(X = 1/4).
3. (26 points) Alice and Bob are given copies of the same text for independent
proofreading. Alice finds 20 errors and Bob finds 15 errors, of which 10 were
found by Alice as well. Assume that Alice and Bob independently detect any
given error with (unknown) probabilities p and p , and that Alice and Bob
A B
each have detected expected numbers of errors. Estimate n which the total
number of errors in the text.
4. (25 points) Professor Whiskey has a class of n students, and he has to hand
back a quiz and a homework assignment. True to form, he hands back the 2n
papers haphazardly, uniformly randomly among all the possible ways to give
each student two items. (Note that he does not necessarily give each student
a quiz and a homework; some people might get two quizzes, others two
homeworks.) Let X be the number of students who end up with their own quiz
and homework. compute E(X) and Var(X).
Hint: You may want to let E be the event that student i gets both of her own
i
items back, and let X be the indicator of E . find P(E ) and P(E |E ) where
i i i j i
j ≠ i.
5. (25 points) If we try a certain experiment n times with a probability p of
success each time, show that the most likely number of successes is k = np.
(Assmue that p is a fraction and n is such that np is an integer.) Use
Stirling's approximation to show that the probability of getting exactly np
successes is
1
P(n, np, p) ≒ ───────
＿＿＿＿＿
√2πnp(1-p)
n! ＿＿
Note that lim ───── = √2π.
n→∞ n+1/2 -n
n e