**Chapter 7 Probability Distributions Ex 7.2**

## Chapter 7 Probability Distributions Ex 7.2

**Question 1.Verify which of the following is p.d.f. of r.v. X:**

**(ii) f(x) = x, for 0 â‰¤ x â‰¤ 1 and -2 â€“ x for 1 < x < 2(iii) fix) = 2, for 0 â‰¤ x â‰¤ 1.Solution:**

f(x) is the p.d.f. of r.v. X if

(a) f(x) â‰¥ 0 for all x âˆˆ R and

Hence, f(x) is the p.d.f. of X.

(ii) f(x) = x â‰¥ 0 if 0 â‰¤ x â‰¤ 1

For 1 < x < 2, -2 < -x < -1

-2 â€“ 2 < -2 â€“ x < -2 â€“ 1

i.e. -4 < f(x) < -3 if 1 < x < 2

Hence, f(x) is not p.d.f. of X.

(iii) (a) f(x) = 2 â‰¥ 0 for 0 â‰¤ x â‰¤ 1

Hence, f(x) is not p.d.f. of X.

**Question 2.The following is the p.d.f. of r.v. X:f(x) = x/8, for 0 < x < 4 and = 0 otherwise.Find(a) P(x < 1.5)(b) P(1 < x < 2) (c) P(x > 2).Solution:**

Question 3.

It is known that error in measurement of reaction temperature (in 0Â°C) in a certain experiment is continuous r.v. given by

= 0. otherwise.

(i) Verify whether f(x) is p.d.f. of r.v. X

(ii) Find P(0 < x â‰¤ 1)

(iii) Find the probability that X is negative.

Solution:

**Question 4.Solution:**

(i) Since, the function f is p.d.f. of X

(ii) Since, the function f is the p.d.f. of X,

**Question 5.Let X be the amount of time for which a book is taken out of the library by a randomly selected students and suppose X has p.d.f.f(x) = 0.5x, for 0 â‰¤ x â‰¤ 2 and = 0 otherwise.Calculate:(i) P(x â‰¤ 1)(ii) P(0.5 â‰¤ x â‰¤ 1.5)(iii) P(x â‰¥ 1.5).Solution:**

(i) P(x â‰¤ 1)

(ii) P(0.5 â‰¤ x â‰¤ 1.5)

(iii) P(x â‰¥ 1.5)

**Question 6.Suppose that X is waiting time in minutes for a bus and its p.d.f. is given by f(x) = 1/5, for 0 â‰¤ x â‰¤ 5 and = 0 otherwise. Find the probability that(i) waiting time is between 1 and 3(ii) waiting time is more than 4 minutes.Solution:**

(i) Required probability = P(1 < X < 3)

(ii) Required probability = P(X > 4)

**Question 7.Suppose the error involved in making a certain measurement is a continuous r.v. X with p.d.f.f(x) = k(4 â€“ ), -2 â‰¤ x â‰¤ 2 and 0 otherwise.Compute:(i) P(X > 0)(ii) P(-1 < X < 1)(iii) P(-0.5 < X or X > 0.5).Solution:**

Since, f is the p.d.f. of X,

**Question 8.The following is the p.d.f. of continuous r.v. Xf(x) = x/8, for 0 < x < 4 and = 0 otherwise.(i) Find expression for c.d.f. of X.(ii) Find F(x) at x = 0.5, 1.7 and 5.Solution:**

(i) Let F(x) be the c.d.f. of X

**Question 9.Given the p.d.f. of a continuous random r.v. X, f(x) = , for -1 < x < 2 and = 0 otherwise. Determine c.d.f. of X and hence find P(X < 1); P(X < -2), P(X > 0), P(1 < X < 2).Solution:**

**Question 10.If a r.v. X has p.d.f.f(x) = c/x for 1 < x < 3, c > 0. Find c, E(X), Var (X).Solution:**
Since f(x) is p.d.f of r.v. X