18.求下列极限:-|||-(1) lim _(xarrow +infty )(dfrac (3n+1)(n)cdot dfrac (n+1)(n));-|||-(2) lim _(narrow +infty )dfrac ({n)^2+3n-1}(3{n)^2+5n-1}-|||-(3) lim _(narrow +infty )sqrt (n)(sqrt (n+1)-sqrt (n));-|||-(4) lim _(narrow +infty )dfrac (1+2+... +n)({n)^2}-|||-(5)lim2 (sinx-cosx-x^2);-|||-2-|||-(6) lim _(xarrow 0)dfrac ({x)^2-1}(2{x)^2-x-1};-|||-(7) lim _(xarrow 1)dfrac ({x)^2-1}(2{x)^2-x-1}-|||-(8) lim _(xarrow 1)(dfrac (1)(1-x)-dfrac (x+2)(1-{x)^3});-|||-(9) lim _(xarrow 16)dfrac (sqrt [4]{x)-2}(sqrt {x)-4};-|||-(10) lim _(xarrow +infty )dfrac (arctan x)(x);-|||-(11) lim _(Delta xarrow 0)dfrac (sqrt {Delta x+1)-1}(Delta x);-|||-(12) lim _(xarrow 1)dfrac (sqrt {3-x)-sqrt (1+x)}({x)^2+x-2}.