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EXERCISE 4.1 · Q9

Q.Prove by method of induction, for all n∈Nn \in N: 13.5+15.7+17.9+… to n terms=n3(2n+3)\dfrac{1}{3.5} + \dfrac{1}{5.7} + \dfrac{1}{7.9} + \ldots \text{ to } n \text{ terms} = \dfrac{n}{3(2n+3)}.

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Let P(n):13.5+15.7+⋯ to n terms=n3(2n+3)P(n):\dfrac1{3.5}+\dfrac1{5.7}+\cdots\text{ to }n\text{ terms}=\dfrac{n}{3(2n+3)}, kkth term 1(2k+1)(2k+3)\dfrac1{(2k+1)(2k+3)}. Base: n=1n=1: L.H.S.=115=\dfrac1{15}, R.H.S.=13(5)=115=\dfrac{1}{3(5)}=\dfrac1{15}; holds. Hypothesis: assume true for kk. Step: add 1(2k+3)(2k+5)\dfrac1{(2k+3)(2k+5)}: $\dfrac{k}{3(2k+3)}+\dfrac1{(2k+3)(2k+5)}=\dfrac{k(2k+5)+3}{3(2k+3)(2k+5)}=\dfrac{2k^2+5k+3}{3(2k+3)(2k+5)}=\dfrac{(2k+3)(k+1)}{3(2k+3)(2k+5)}=\dfrac{k+1}{3(2k+5)} …

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