21. (See exercise 7.) If an annual payment annuity of 100 is to be received from time n þ 1 to time n þ m, show that the value of this n-year deferred, m-year annuity can be represented as either of the following:

(a) 100ðanþm; r  an; rÞ

(b) 100ð1 þ rÞnam; r