programminga general overview series the easiest formulas to remember are + = and the companionn ai = an+ - in the latter formulaif then ai < = - and as tends to the sum approaches /( athese are the "geometric seriesformulas we can derive the last formula for = ( in the following manner let be the sum then then as if we subtract these two equations (which is permissible only for convergent series)virtually all the terms on the right side cancelleaving as which implies that - we can use this same technique to compute = / sum that occurs frequently we write ** and multiply by obtaining ** subtracting these two equations yields = thuss **