lim(x,y)趋近于(0,0)[1-cos(x^2+y^2)]/(x^2+y^2)expx^2y^2
=lim(x,y)趋近于(0,0)[1-cos(x^2+y^2)]/(x^2+y^2)
=lim(x趋近于0) (1-cosx)/x
=lim(x趋近于0) sinx/1
=0