Biết \[f(x + y,x - y) = xy\]. Tìm \[f(x,y)\]

\[f\left( {x,y} \right) = \frac{{{x^2} - {y^2}}}{4}\]

\[f\left( {x,y} \right) = \frac{{{x^2} + {y^2}}}{4}\]

\[f\left( {x,y} \right) = \frac{{ - {x^2} + {y^2}}}{4}\]

\[f\left( {x,y} \right) = \frac{{{x^2} - {y^2}}}{4}\]