1.试求下列极限(包括非正常极限):-|||-(1) lim _((x,y))(0,0)dfrac ({x)^2(y)^2}({x)^2+(y)^2} ;-|||-(2) lim _((x,y)arrow (0,0))dfrac (1+{x)^2+(y)^2}({x)^2+(y)^2} ;-|||-(3) lim _((x,y)arrow (0,0))dfrac ({x)^2+(y)^2}(sqrt {1+{x)^2+(y)^2}-1} ;-|||-(4) lim _((x,y)arrow (0,0))dfrac (xy+1)({x)^4+(y)^4} ;-|||-(5) lim _((x,y)arrow (1,2))dfrac (1)(2x-y)-|||-(6) lim _((x,y)arrow (0,0))(x+y)sin dfrac (1)({x)^2+(y)^2} ;-|||-(7) lim _((x,y)arrow (0,0))dfrac (sin ({x)^2+(y)^2)}({x)^2+(y)^2}