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Algebra Difficulty 5.2 AIME, harder Prove it Croatia

Prove that
log2(1+11)+log2(1+12)++log2(1+1k)++log2(1+12014)<11. \log_2 \left(1 + \frac{1}{1}\right) + \log_2 \left(1 + \frac{1}{2}\right) + \dots + \log_2 \left(1 + \frac{1}{k}\right) + \dots + \log_2 \left(1 + \frac{1}{2014}\right) < 11.

Solution

Let us denote the sum by SS:
S=log2(1+11)+log2(1+12)++log2(1+12014) S = \log_2 \left(1 + \frac{1}{1}\right) + \log_2 \left(1 + \frac{1}{2}\right) + \dots + \log_2 \left(1 + \frac{1}{2014}\right)

We have:
log2(1+1k)=log2(k+1k)=log2(k+1)log2k \log_2 \left(1 + \frac{1}{k}\right) = \log_2 \left(\frac{k+1}{k}\right) = \log_2 (k+1) - \log_2 k

Therefore,
S=k=12014[log2(k+1)log2k] S = \sum_{k=1}^{2014} \left[\log_2 (k+1) - \log_2 k\right]

This is a telescoping sum:
S=log22log21+log23log22+log24log23++log22015log22014 S = \log_2 2 - \log_2 1 + \log_2 3 - \log_2 2 + \log_2 4 - \log_2 3 + \dots + \log_2 2015 - \log_2 2014

All terms except the first log22\log_2 2 and the last log22014-\log_2 2014 cancel, so:
S=log2(2015)log21=log2(2015) S = \log_2 (2015) - \log_2 1 = \log_2 (2015)

We need to show that log2(2015)<11\log_2 (2015) < 11.

But 211=2048>20152^{11} = 2048 > 2015, so log2(2015)<11\log_2 (2015) < 11.

Therefore,
log2(1+11)+log2(1+12)++log2(1+12014)<11. \log_2 \left(1 + \frac{1}{1}\right) + \log_2 \left(1 + \frac{1}{2}\right) + \dots + \log_2 \left(1 + \frac{1}{2014}\right) < 11.

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Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.