3.求下列极限:-|||-(1) lim _(xarrow 2)dfrac ({x)^2-3x-4}({x)^2-4} :-|||-(3) lim _(xarrow infty )dfrac (3{x)^2+5x+1}({x)^2+3x+4} :-|||-(5) lim _(xarrow infty )(1+dfrac (1)(x))(2-dfrac (1)({x)^2}) ;-|||-(7) lim _(xarrow 1)dfrac (sqrt {3-x)-sqrt (1+x)}({x)^2-1} ;-|||-(9) lim _(xarrow 1)dfrac ({x)^m-1}({x)^n-1} (m,n是正整数);-|||-(11) lim _(xarrow 0)dfrac ((1+x)(1+2x)(1+3x)-1)(x)-|||-(2) lim _(harrow 0)dfrac ({(x+h))^3-(x)^3}(h) ;-|||-(4) lim _(xarrow infty )dfrac ({(2x-3))^20((3x+2))^30}({(5x+1))^50}-|||-(6) lim _(xarrow infty )dfrac (2{x)^3+3x+1}(4{x)^5+2x+7} ;-|||-(8)lim _(xarrow 1)(dfrac (1)(1-x)-dfrac (3)(1-{x)^3});-|||-(10)lim √x-1-|||-(12) lim _(xarrow +infty )(sqrt ((x+2)(x-1))-x) .