Determine the value of 2002+21(2001+21(2000+⋯+21(3+21⋅2))⋯)
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+21([n−1]+21(⋯+21⋅2)⋯)=2(n−1). For n=3 we have 3+21⋅2=4, giving the base case, and if the result holds for n, then (n+1)+212(n−1)=2n=2(n+1)−2. Thus the claim holds, and now plug in n=2002. Alternate Solution: Expand the given expression as 2002+2001/2+2000/22+⋯+2/22000. Letting S denote this sum, we have S/2=2002/2+2001/22+⋯+2/22001, so S−S/2=2002−(1/2+1/4+⋯+1/22000)−2/22001=2002−(1−1/22000)−1/22000=2001, so S=4002.
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