P (It will rain tomorrow) = 1 - P (It will not rain tomorrow) true or false

The answer is TRUE

There are only two possible outcomes

- Tomorrow it rains.

- Tomorrow it does not rain.

Thus the probability of one outcome dictates the probability of the other.

For example, imagine that

P (It will rain tomorrow) = 1 (This means that you are 100% sure it will rain)

Then you can assure that

P (It will rain tomorrow) = 1 - P (It will not rain tomorrow)

P (It will not rain tomorrow) = 0