If sin(x+y)sin(xy)=2f(x)siny,\displaystyle {\sin{{\left({x}+{y}\right)}}}-{\sin{{\left({x}-{y}\right)}}}={2}{f{{\left({x}\right)}}}{\sin{{y}}}, then
f(x)=\displaystyle {f{{\left({x}\right)}}}=   .