[题目](1) dfrac {1)({2)^n}} -|||-(2) {(-1))^ndfrac (1)(n)} -|||-(3) 2+dfrac {1)({n)^2}} -|||-(4) dfrac {n-1)(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)}