求证明期货、期权及其他衍生品那本书上的一个公式
题目如下：
Prove the following formula (Hint: equate the final amount available when (a)investing $1 at R2 for T2 years and  (b) investing $1 at R1 for T1 years, followed by (T2-T1) years at Rf:

Rf=(R2T2-R1T1)/(T2-T1)

求大神指教~~~~