Let f(x,y)=xyx2−y2x2+y2f(x,y)=xy\dfrac {x^{2}-y^{2}}{x^{2}+y^{2}}f(x,y)=xyx2+y2x2−y2, if x2+y2≠0x^{2}+y^{2} \neq 0x2+y2=0 and f(0,0)=0f(0,0)=0f(0,0)=0. Prove that