Show that V1 + f = V2, where V1 is the value of a swaption to pay a fixed rate of sK and receive LIBOR between times T1and T2, f is the value of a forward swap to receive a fixed rate of sK and pay LIBOR between times T1 and T2, and V2 is the value of a swaption to receive a fixed rate of sK between times T1 and T2.
Deduce that V1 = V2 when sK equals the current forward swap rate.