Question

explain impossible event and sure event

Answer 1

Impossible event is an event which does not occur in the sample space of an experiment. It is, well, impossible. Its probability is 0. For example, getting 7 in a single throw of a fair die.
Sure event is an event which is going to happen in the sample space everytime, no matter what. Its probability is 1. For example, getting less than 7 in a single throw of a fair die.

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Answer 2

An impossible event is the opposite case, when the eventdoes not contain any element of the sample space.
 A sure event is the one that contains the whole sample space.