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Algebra Difficulty 4.6 AIME Prove it

Use mathematical induction to prove that 122+dfrac132+ldots+dfrac1(n+1)2>dfrac12dfrac1n+2\frac{{1}}{{{{2}}^{{2}}}}+\\dfrac{{1}}{{{{3}}^{{2}}}}+\\ldots +\\dfrac{{1}}{{{(n+{1})}^{{2}}}} > \\dfrac{{1}}{{2}}-\\dfrac{{1}}{n+{2}}. Assume that the inequality holds when n=kn=k, then the target inequality to be proved when n=k+1n=k+1 is ________ .

Solution

Step 1: Understanding the Problem
The problem asks us to use mathematical induction to find the target inequality when n=k+1n=k+1, given that the inequality holds when n=kn=k.

Step 2: Applying Mathematical Induction
Assume that the inequality holds when n=kn=k, i.e., 122+dfrac132+ldots+dfrac1(k+1)2>dfrac12dfrac1k+2\frac{{1}}{{{{2}}^{{2}}}}+\\dfrac{{1}}{{{{3}}^{{2}}}}+\\ldots +\\dfrac{{1}}{{{(k+{1})}^{{2}}}} > \\dfrac{{1}}{{2}}-\\dfrac{{1}}{k+{2}}.

When n=k+1n=k+1, the left side of the inequality becomes 122+dfrac132+ldots+dfrac1(k+1)2+1(k+2)2\frac{{1}}{{{{2}}^{{2}}}}+\\dfrac{{1}}{{{{3}}^{{2}}}}+\\ldots +\\dfrac{{1}}{{{(k+{1})^{2}}}} + \frac{1}{(k+2)^2}.

Step 3: Simplifying the Inequality
According to our assumption, we know that 122+dfrac132+ldots+dfrac1(k+1)2>dfrac12dfrac1k+2\frac{{1}}{{{{2}}^{{2}}}}+\\dfrac{{1}}{{{{3}}^{{2}}}}+\\ldots +\\dfrac{{1}}{{{(k+{1})^{2}}}} > \\dfrac{{1}}{{2}}-\\dfrac{{1}}{k+{2}}.

Adding 1(k+2)2\frac{1}{(k+2)^2} to both sides of the inequality gives us 122+dfrac132+ldots+dfrac1(k+1)2+1(k+2)2>dfrac12dfrac1k+2+1(k+2)2\frac{{1}}{{{{2}}^{{2}}}}+\\dfrac{{1}}{{{{3}}^{{2}}}}+\\ldots +\\dfrac{{1}}{{{(k+{1})^{2}}}} + \frac{1}{(k+2)^2} > \\dfrac{{1}}{{2}}-\\dfrac{{1}}{k+{2}} + \frac{1}{(k+2)^2}.

Step 4: Final Inequality
Therefore, when n=k+1n=k+1, the target inequality to be proved is 122+dfrac132+ldots+dfrac1(k+1)2+1(k+2)2>dfrac12dfrac1k+2+1(k+2)2\boxed{\frac{{1}}{{{{2}}^{{2}}}}+\\dfrac{{1}}{{{{3}}^{{2}}}}+\\ldots +\\dfrac{{1}}{{{(k+{1})^{2}}}} + \frac{1}{(k+2)^2} > \\dfrac{{1}}{{2}}-\\dfrac{{1}}{k+{2}} + \frac{1}{(k+2)^2}}.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.