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用数学归纳法证明:\( \dfrac{{1}^{2}}{1-3}+ \dfrac{{2}^{2}}{3-5}+...+ \dfrac{{n}^{2}}{(2n-1)(2n+1)}= \dfrac{n(n+1)}{2(2n+1)} \)
用数学归纳法证明\({1}^{2}+{2}^{2}+…+{\left(n-1\right)}^{2}+{n}^{2}+{\left(n-1\right)}^{2}+…+{2}^{2}+{1}^{2}= \dfrac{n\left(2{n}^{2}+1\right)}{3} \)时,由\(n=k\)的假设到证明\(n=k+1\)时,等式左边应添加的式子是\((\) \()\)
用数学归纳法证明:\(1^{2}+2^{2}+3^{2}+…+n^{2}+…+2^{2}+1^{2}= \dfrac{n\left(2{n}^{2}+1\right)}{3} \),第二步证明由\(n=k\)到\(n=k+1\)时,左边应加\((\) \()\)
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