IMO Shortlist 2002 problem A4


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2. travnja 2012.
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Find all functions f from the reals to the reals such that \left(f(x)+f(z)\right)\left(f(y)+f(t)\right)=f(xy-zt)+f(xt+yz) for all real x,y,z,t.
Izvor: Međunarodna matematička olimpijada, shortlist 2002