NCERT Solutions
Class 12 Maths
Probability

Ex.13.4 Q.13
Let X denotes the sum of the numbers obtained when two fair dice are rolled. Find the variance and standard deviation of X.
When two fair dice are rolled, 6 × 6 = 36 observations are obtained.
P(X = 2) = P(1, 1)
P(X = 3) = P (1, 2) + P(2, 1)
P(X = 4) = P(1, 3) + P(2, 2) + P(3, 1)
P(X = 5) = P(1, 4) + P(2, 3) + P(3, 2) + P(4, 1)
P(X = 6) = P(1, 5) + P (2, 4) + P(3, 3) + P(4, 2) + P(5, 1)
P(X = 7) = P(1, 6) + P(2, 5) + P(3, 4) + P(4, 3) + P(5, 2) + P(6, 1)
P(X = 8) = P(2, 6) + P(3, 5) + P(4, 4) + P(5, 3) + P(6, 2)
P(X = 9) = P(3, 6) + P(4, 5) + P(5, 4) + P(6, 3) =
P(X = 10) = P(4, 6) + P(5, 5) + P(6, 4)
P(X = 11) = P(5, 6) + P(6, 5)
P(X = 12) = P(6, 6)
Therefore, the required probability distribution is as follows:
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X |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
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P(X) |
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Then, E(X) = Σ Xi × P(Xi)
Now, E(X) = Σ X2i × P(Xi)
Again, Var(X) = E[X2] – [E(X)]2
= 54.833 – 72
= 54.833 – 49
= 5.833
So, Standard deviation = √[Var(X)]
= √(5.833)
= 2.415