)求下列极限:-|||-(1) lim _(xarrow 1)dfrac ({x)^2-x+1}({(x-1))^2} =-|||-(3) lim _(xarrow infty )((dfrac {2x+3)(2x+1))}^x+1 ;-|||-(5) lim _(xarrow 0)((dfrac {{a)^x+(b)^x+(c)^x}(3))}^dfrac (1{x)}(agt 0,bgt 0,cgt 0) ;-|||-(7) lim _(xarrow a)dfrac (ln x-ln a)(x-a)(agt 0) ;-|||-(2) lim _(xarrow +infty )x(sqrt ({x)^2+1}-x) ;-|||-(4) lim _(xarrow 0)dfrac (tan x-sin x)({x)^3} :-|||-((6)lim(sin x)^1anx;-|||-arrow dfrac (pi )(2)-|||-(8) lim _(xarrow 0)dfrac (xtan x)(sqrt {1-{x)^2}-1}