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Inequalities Simplified Revision Notes

Revision notes with simplified explanations to understand Inequalities quickly and effectively.

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Inequalities

Mathematical induction can be used to prove that a quantity is greater or low than another quantity.

Example

infoNote

Prove by induction that for all n≥1n \geq 1.

2n>n22^n > n^2

First, prove this proposition is true for the base case n=1n=1.

21=2and12=1.2^1 = 2 \quad \text{and} \quad 1^2 = 1.

True for base case, n=1n=1. Clearly 2>12>1.


Next, assume true for some arbitrary number kk. Assume true for n=kn=k :

2k>k22^k > k^2

is true for k≥1k \geq 1.


Finally, prove for n=k+1n=k+1.

2k+1>(k+1)2.2^{k+1} > (k+1)^2.

Use the definition for the powers of 22.

2k+1=2â‹…2k.2^{k+1} = 2 \cdot 2^k.

By the inductive hypothesis, 2k>k22^k > k^2, so:

2k+1=2â‹…2k>2â‹…k2.2^{k+1} = 2 \cdot 2^k > 2 \cdot k^2.

Now compare 2â‹…k22 \cdot k^2 and (k+1)2(k + 1)^2:

(k+1)2=k2+2k+1.(k + 1)^2 = k^2 + 2k + 1.

We need to show :

2â‹…k2>k2+2k+1,2 \cdot k^2 > k^2 + 2k + 1,

which simplifies to :

k2>2k+1.k^2 > 2k + 1.

By mathematical induction, 2n>n22^n > n^2 holds for all n≥1n \geq 1.

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