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Algebra Difficulty 5.2 AIME, harder Find the answer

Determine the value of 2002+12(2001+12(2000++12(3+122)))2002+\frac{1}{2}\left(2001+\frac{1}{2}\left(2000+\cdots+\frac{1}{2}\left(3+\frac{1}{2} \cdot 2\right)\right) \cdots\right)

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

Solution

We can show by induction that n+12([n1]+12(+122))=2(n1)n+\frac{1}{2}\left([n-1]+\frac{1}{2}\left(\cdots+\frac{1}{2} \cdot 2\right) \cdots\right)=2(n-1). For n=3n=3 we have 3+122=43+\frac{1}{2} \cdot 2=4, giving the base case, and if the result holds for nn, then (n+1)+122(n1)=2n=2(n+1)2(n+1)+\frac{1}{2} 2(n-1)=2 n=2(n+1)-2. Thus the claim holds, and now plug in n=2002n=2002. Alternate Solution: Expand the given expression as 2002+2001/2+2000/22++2/220002002+2001 / 2+2000 / 2^{2}+\cdots+2 / 2^{2000}. Letting SS denote this sum, we have S/2=2002/2+2001/22++2/22001S / 2=2002 / 2+2001 / 2^{2}+\cdots+2 / 2^{2001}, so SS/2=S-S / 2= 2002(1/2+1/4++1/22000)2/22001=2002(11/22000)1/22000=20012002-\left(1 / 2+1 / 4+\cdots+1 / 2^{2000}\right)-2 / 2^{2001}=2002-\left(1-1 / 2^{2000}\right)-1 / 2^{2000}=2001, so S=4002S=4002.

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