# [Discrete Probability Function] P(X²=1)= P(X=1) U P(X=-1)?

#### Ardêncio

##### New Member
I would like to know if this following form of interpretation is correct:

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Suppose I have a discrete probability function P(X=k)=(7-k)/21, k between [1,6].
The question asks me to calculate P(X²=1).

In this case, may I do P(X²=1)= P(X=1) U P(X=-1)?

#### Dason

##### Ambassador to the humans
Close. I mean... you're right but your notation is a little off. You would want P(X²=1)= P(X=1 U X=-1) = P(X=1) + P(X = -1) since they're disjoint.