note that ∫∫f(x,y)dxdy is a constant,let it be c,thenxy*c^2=f(x,y)-1f(x,y)=xy*c^2+1,take the integral,we get∫∫f(x,y)dxdy=(c^2)/4+1but as assumed,it equals c.solve:(c^2)/4+1=c we get c=2thus,f(x,y)...