证明函数x^2+y^2≠0时,f(x,y)=sin(xy)/√(x^2+y^2),x^2+y^2=0时f(x,y)=0在(0,0)处连续

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证明函数x^2+y^2≠0时,f(x,y)=sin(xy)/√(x^2+y^2),x^2+y^2=0时f(x,y)=0在(0,0)处连续