Op-amp problem....

I searched three detailed books that had hundreds of pages about op-amps and I can't find the solution to this problem. I think there's a simple solution but I just can't work it out..

Here's the schematic of the op-amp differential amplifier: : R2 ---------/\/\/\/\/\---------- | | | | | | | \ | R1 | \ | Vi_1 --------------/\/\/\/\/\--------- - \ | | ----------------------| V_o R3 | / Vi_2 --------------/\/\/\/\/\---------- + / | | / | | R4 \-----/\/\/\/\/\----- | | Gnd

The first part is to give V_o in terms of the other variables, which is easy enough and is in the books. The next part is:

Vi_1(t) = 10^(-3) Sin(2pi x 10^3t) + Vn(t) Vi_2(t) = 10^(-3) Cos(2pi x 10^3t) + Vn(t)

R_1 = R_2 = 1kohm R_2 = R_4 =

30kohm

where Vn(t) denotes unwanted noise. If the Common Mode Rejection Ratio (CMMR) of the op-amp is 97 dB and both inputs are "contaminated" with

50 Hz noise of average amplitude 1V, calculate the ratio of output signal amplitude to average output noise amplitude.

I know what the CMMR equation is but the frequencies are confusing me...