下列各题中,哪些数列收敛,哪些数列发散?对收敛数列,通过观察(xn)的变化-|||-趋势,写出它们的极限:-|||-(1) dfrac {1)({2)^n}} ;-|||-(2) {(-1))^ndfrac (1)(n)} ;-|||-(3) 2+dfrac {1)({n)^2}} ;-|||-(4)[n+1]-|||-(5) n{(-1))^n} ;-|||-(6) dfrac {{2)^n-1}({3)^n}} -|||-(7) n-dfrac {1)(n)} ;-|||-(8) [ {(-1))^n+1] dfrac (n+1)(n)}