已知数列｛an｝满足 1/an - an =2根号n,且an>0 求数列｛an｝的通项公式,2.还有证明,之和小于根号n3.数列｛an｝是否存在最大项?若存在最大项求出该项和相应的项数；若存在,求出；最少第一问就

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