Complementary events must have a sum of 1.

This is a true statement.

Consider two events A and B. We say they are complementary if P(A)+P(B) = 1

This means that either event A or event B must happen, since the "1" represents 100% probability. Having a probability of 100% means absolute certainty.

Example:

A = the event it rains

B = the event it does not rain

P(A) = 0.30 = 30% chance of rain

P(B) = 0.70 = 70% chance it does not rain

P(A)+P(B) = 0.30+0.70 = 1

So this shows the two events are complementary.

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