( 1 − 2 2 1 ) ( 1 − 3 2 1 ) ( 1 − 4 2 1 ) ⋯ ( 1 − 2 0 1 7 2 1 )
If the above product can be expressed as 2 b a , where a and b are coprime positive integers, then find a + b .
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Did you mean ∏ k = 2 2 0 1 7
Hey, this problem is similar like mine. And I post it earlier than you.. See this problem
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Aw man, sorry about that! I hadn't seen that problem of yours before. I just happened to think of it, and stopped at 2 0 1 7 because that was the year!
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No worries man.. It doesn't even matter, this can be happen anytime..
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Relevant wiki: Telescoping Series - Product
1 − k 2 1 = k 2 k 2 − 1 = k 2 ( k − 1 ) ( k + 1 )
Thus,
( 1 − 2 2 1 ) ( 1 − 3 2 1 ) ⋯ ( 1 − 2 0 1 7 2 1 ) = k = 2 ∏ 2 0 1 7 k 2 ( k − 1 ) ( k + 1 )
2 2 1 ⋅ 3 ⋅ 3 2 2 ⋅ 4 ⋅ 4 2 3 ⋅ 5 ⋯ ⋯ 2 0 1 6 2 2 0 1 5 ⋅ 2 0 1 7 ⋅ 2 0 1 7 2 2 0 1 6 ⋅ 2 0 1 8 = 2 ⋅ 2 0 1 7 2 0 1 8