What must be true in order for p (a or b) = p(a) + (b)
a and b cannot be disjoint
a and b must be independent
a and b cannot be independent
a and b must be disjoint
it is always true

A and B must be disjoint. You have, by the inclusion/exclusion principle, that



where the last term is required to prevent double counting. When and are disjoint, you have and , leaving you with

The lateral side of a triangular pyramid is a triangle. in order to increase the lateral surface area, the slant height and the height should be altered