AlgebraDifficulty 3.8AMC 10/12Find the answerEstonia
Find the sum 1+121+221+1+221+321+⋯+1+202121+202221
A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.
Solution
We will denote s=1+121+221+1+221+321+⋯+1+202121+202221 Notice that 1+k21+(k+1)21=k2(k+1)2k2(k+1)2+(k+1)2+k2=(k(k+1))2(k(k+1)+1)2, which yields 1+k21+(k+1)21=k(k+1)k(k+1)+1=1+k(k+1)1=1+k1−k+11. Therefore s = 2021 + (11−21)+(21−31)+⋯+(20211−20221) = 2021 + 20222021.
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