<Dd> a, a r, a r 2, a r 3, a r 4,...(\ displaystyle a, \ ar, \ ar ^ (2), \ ar ^ (3), \ ar ^ (4), \ \ ldots) </Dd> <P> where r ≠ 0 is the common ratio and a is a scale factor, equal to the sequence's start value . </P> <P> The n - th term of a geometric sequence with initial value a and common ratio r is given by </P> <Dl> <Dd> a n = a r n − 1 . (\ displaystyle a_ (n) = a \, r ^ (n - 1).) </Dd> </Dl>

P(a or b ) = p(a) + p(b) - p(a and b) geometrically explain why we need the last term