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Algebra Difficulty 3.8 AMC 10/12 Find the answer Estonia

Find the sum
1+112+122+1+122+132++1+120212+120222 \sqrt{1 + \frac{1}{1^2} + \frac{1}{2^2}} + \sqrt{1 + \frac{1}{2^2} + \frac{1}{3^2}} + \dots + \sqrt{1 + \frac{1}{2021^2} + \frac{1}{2022^2}}

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

We will denote
s=1+112+122+1+122+132++1+120212+120222 s = \sqrt{1 + \frac{1}{1^2} + \frac{1}{2^2}} + \sqrt{1 + \frac{1}{2^2} + \frac{1}{3^2}} + \dots + \sqrt{1 + \frac{1}{2021^2} + \frac{1}{2022^2}}
Notice that
1+1k2+1(k+1)2=k2(k+1)2+(k+1)2+k2k2(k+1)2=(k(k+1)+1)2(k(k+1))2, 1 + \frac{1}{k^2} + \frac{1}{(k+1)^2} = \frac{k^2(k+1)^2 + (k+1)^2 + k^2}{k^2(k+1)^2} = \frac{(k(k+1)+1)^2}{(k(k+1))^2},
which yields
1+1k2+1(k+1)2=k(k+1)+1k(k+1)=1+1k(k+1)=1+1k1k+1. \sqrt{1 + \frac{1}{k^2} + \frac{1}{(k+1)^2}} = \frac{k(k+1)+1}{k(k+1)} = 1 + \frac{1}{k(k+1)} = 1 + \frac{1}{k} - \frac{1}{k+1}.
Therefore

s = 2021 + (1112)+(1213)++(1202112022)\left(\frac{1}{1} - \frac{1}{2}\right) + \left(\frac{1}{2} - \frac{1}{3}\right) + \dots + \left(\frac{1}{2021} - \frac{1}{2022}\right) = 2021 + 20212022.\frac{2021}{2022}.

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