Using the principle of mathematical induction, prove that dnadxn=(−1)nn!(x+5)n+1\frac{d^{n}a}{dx^{n}}=\frac{(-1)^{n}n!}{(x+5)^{n+1}}dxndna=(x+5)n+1(−1)nn! for a=1x+5a=\frac{1}{x+5}a=x+51.
Just explain me the inductive step! Thanks :)
Note by Omkar Kulkarni 6 years, 1 month ago
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CASE I (n=1n=1n=1): ddx1x+5=−1(x+5)2=(−1)1⋅1!(x+5)1+1⇒\large \frac{d}{dx} \frac{1}{x+5} = \frac{-1}{(x+5)^2} = \frac{(-1)^{1} \cdot 1!}{(x+5)^{1+1}} \Rightarrow dxdx+51=(x+5)2−1=(x+5)1+1(−1)1⋅1!⇒ TRUE.
CASE II (n=kn=kn=k): dkdxk1x+5=(−1)k⋅k!(x+5)k+1⇒\large \frac{d^k}{dx^k} \frac{1}{x+5} = \frac{(-1)^k \cdot k!}{(x+5)^{k+1}} \Rightarrowdxkdkx+51=(x+5)k+1(−1)k⋅k!⇒ ASSUMED TRUE.
CASE III (n=k+1n=k+1n=k+1): ddx[dkdxk1x+5]=ddx[(−1)k⋅k!(x+5)k+1];\large \frac{d}{dx}[\frac{d^k}{dx^k} \frac{1}{x+5}] = \frac{d}{dx}[\frac{(-1)^k \cdot k!}{(x+5)^{k+1}}];dxd[dxkdkx+51]=dxd[(x+5)k+1(−1)k⋅k!];
or dk+1dxk+11x+5=(−1)k⋅−(k+1)⋅k!(x+5)k+2;\large \frac{d^{k+1}}{dx^{k+1}} \frac{1}{x+5}= \frac{(-1)^k \cdot -(k+1) \cdot k!}{(x+5)^{k+2}};dxk+1dk+1x+51=(x+5)k+2(−1)k⋅−(k+1)⋅k!;
or dk+1dxk+11x+5=(−1)k+1⋅(k+1)!(x+5)(k+1)+1⇒\large \frac{d^{k+1}}{dx^{k+1}} \frac{1}{x+5} = \frac{(-1)^{k+1} \cdot (k+1)!}{(x+5)^{(k+1)+1}} \Rightarrowdxk+1dk+1x+51=(x+5)(k+1)+1(−1)k+1⋅(k+1)!⇒ TRUE.
Q.E.D. \mathbb{Q.} \mathbb{E.} \mathbb{D.}Q.E.D.
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CASE I (n=1): dxdx+51=(x+5)2−1=(x+5)1+1(−1)1⋅1!⇒ TRUE.
CASE II (n=k): dxkdkx+51=(x+5)k+1(−1)k⋅k!⇒ ASSUMED TRUE.
CASE III (n=k+1): dxd[dxkdkx+51]=dxd[(x+5)k+1(−1)k⋅k!];
or dxk+1dk+1x+51=(x+5)k+2(−1)k⋅−(k+1)⋅k!;
or dxk+1dk+1x+51=(x+5)(k+1)+1(−1)k+1⋅(k+1)!⇒ TRUE.
Q.E.D.