If A and B are two complementary events then the relation between P(A) and P(B) is :

1. P(A) = P(B)

2. P(A) + P(B) = 0

3. P(A) + P(B) = 1

4. none of these

We know that,

The probabilities of two complimentary events add up to 1.

∴ P(A) + P(B) = 1

Hence, Option 3 is the correct option.