5.应用柯西收敛准则,证明以下数列(an)收敛:-|||-(1) _(n)=dfrac (sin 1)(2)+dfrac (sin 2)({2)^2}+... +dfrac (sin n)({2)^n} ;-|||-(2) _(n)=1+dfrac (1)({2)^2}+dfrac (1)({3)^2}+... +dfrac (1)({n)^2}