15.计算下列极限:-|||-(1) lim _(xarrow 0)sqrt ({x)^2-4x+2} ;-|||-(3) lim _(xarrow 0)dfrac (sqrt {4-x)-2}(x) ;-|||-(5) lim _(xarrow infty )(e)^dfrac (1{x)} ;-|||-(7) lim _(xarrow dfrac {pi )(6)}dfrac (sin x-cos x)(cos 2x)-|||-(9) lim _(xarrow 0)((1+3{tan )^2x)}^(cot ^2x) ;-|||-(2) lim _(xarrow -2)dfrac (2{x)^2+1}(x+1) ;-|||-(4) lim _(tarrow 3)ln ((t)^2-1) ;-|||-(6) lim _(xarrow 0)dfrac (sqrt {4+x)-2}(sin 2x) ;-|||-(8) lim _(harrow 0)dfrac (sqrt {x+h)-sqrt (x)}(h)-|||-(10) lim _(xarrow 0)dfrac ({e)^x-1}(x)