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Algebra Difficulty 5.6 AIME, harder Prove it Brazil

Find all functions f:RRf: \mathbb{R} \to \mathbb{R} such that
f(x)f(y)+f(x)=2f(x)+xy f(x)f(y) + f(x) = 2f(x) + xy
for every real x,yx, y.

Solution

First, ff is bijective because, plugging x=1x = 1, we get f(f(y)+f(1))=2f(1)+xf(f(y) + f(1)) = 2f(1) + x; x+2f(1)x + 2f(1) spans all real numbers (so ff is surjective) and f(x)=f(y)    f(x)+f(1)=f(y)+f(1)    f(f(x)+f(1))=f(f(y)+f(1))    2f(1)+x=2f(1)+y    x=yf(x) = f(y) \iff f(x) + f(1) = f(y) + f(1) \iff f(f(x) + f(1)) = f(f(y) + f(1)) \iff 2f(1) + x = 2f(1) + y \iff x = y (so ff is injective).
Choose x0x \neq 0 and yy such that 2f(x)+xy=f(x)    y=f(x)x2f(x) + xy = f(x) \iff y = -\frac{f(x)}{x}. By injectivity,
xf(f(x)x)+f(x)=x    f(f(x)x)+f(x)x=1() x f\left(-\frac{f(x)}{x}\right) + f(x) = x \iff f\left(-\frac{f(x)}{x}\right) + \frac{f(x)}{x} = 1 \quad (*)
Now, set x=y=0x = y = 0: f(f(0))=2f(0)f(f(0)) = 2f(0). Let a=f(0)a = f(0), so that f(a)=2af(a) = 2a. If a=0a = 0, setting y=0y = 0 we get f(f(x))=2f(x)f(f(x)) = 2f(x) and by surjectivity f(x)=2xf(x) = 2x for all xx. This function doesn't work (by testing), so aa can't be 0. So a0a \neq 0 and plugging x=ax = a in ()(*), we obtain (recall that f(a)=2af(a) = 2a):
f(2)+2=1    f(2)=1 f(-2) + 2 = 1 \iff f(-2) = -1
Now set y=0y = 0: f(f(x)x)=2(f(x)x)f(f(x) - x) = 2(f(x) - x). Now we find all real numbers kk such that f(k)=2kf(k) = 2k. If there is only one possible value for kk, we are almost done because f(x)x=k    f(x)=x+kf(x) - x = k \iff f(x) = x + k and by testing the function, f(x)=x+1f(x) = x + 1.
Let mm be such that f(m)=kf(m) = k (it exists because ff is surjective). Plug x=mx = m and y=0y = 0: recalling that f(0)=af(0) = a,
f(mf(0)+f(m))=2f(m)    f(ma+k)=2k=f(k) f(mf(0) + f(m)) = 2f(m) \iff f(ma + k) = 2k = f(k)
By injectivity,
ma+k=k    ma=0    m=0 ma + k = k \iff ma = 0 \iff m = 0
because we already proved that a0a \neq 0. So there is only one value for kk, namely, k=f(0)=ak = f(0) = a. Then, we're done: the only function is f(x)=x+1f(x) = x + 1.

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