How can I distinguish the composite wave with DFT?

Hi,everyone,
Happy New Year!
I have a wave y(t) composed of two cosine waves and white noise

y(t)*cos(w1*t+phi1)+A2*cos(w2*t+phi2)+w(t);
phi1 and phi2 are phases;
first, let w(t)~N(0,sigma^2)and white;
sampling frequency fs>=2*f2, f1 given fs: vary A1,A2,sigma^2, and w1,w2
find how small the difference of w1-w2, using DFT,
where the y(t) is indistinguishable?

it seems difficult to do it. can some one help me with this problem?
any input is greatly appreciated!

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