6.求下列各极限:-|||-(1) lim _((x,y)arrow (0,1))dfrac (1-xy)({x)^2+(y)^2} :-|||-(3) lim _((x,y)arrow (0,0))dfrac (2-sqrt {xy+4)}(xy) =-|||-(5) lim _((x,y)arrow (2,0))dfrac (tan (xy))(y) :-|||-(2) lim _((x,y)arrow (1,0))dfrac (ln (x+{e)^y)}(sqrt {{x)^2+(y)^2}} :-|||-(4) lim _((x,y)arrow (0,0))dfrac (xy)(sqrt {2-{e)^xy}-1} ;-|||-(6) lim _((x,y)arrow (0,0))dfrac (1-cos ({x)^2+(y)^2)}(({x)^2+(y)^2)(e)^(x^2{y)^2} -