1 − 5 − 1 3 − 2 5 − 4 1 − 6 1 − 8 5 − ⋯ 1 2 9 6 6 2 5 2 5 6 8 1 1 6 1 1 = A π B
where A and B are rational numbers. Find A B .
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By using Euler's continued fraction formula , we change the fraction into another form by expanding or reducing fractions and the formula. 1 − 5 − 1 3 − 2 5 − 4 1 − 6 1 − 8 5 − ⋯ 1 2 9 6 6 2 5 2 5 6 8 1 1 6 1 1 = 1 − 4 5 − 9 1 3 − 1 6 2 5 − 2 5 4 1 − 3 6 6 1 − 4 9 8 5 − ⋯ 4 9 3 6 3 6 2 5 2 5 1 6 1 6 9 9 4 4 1 1 = 1 + 1 × 4 1 + 1 × 4 1 × 9 4 + 1 × 4 1 × 9 4 × 1 6 9 + 1 × 4 1 × 9 4 × 1 6 9 × 2 5 1 6 + 1 × 4 1 × 9 4 × 1 6 9 × 2 5 1 6 × 3 6 2 5 + 1 × 4 1 × 9 4 × 1 6 9 × 2 5 1 6 × 3 6 2 5 × 4 9 3 6 + ⋯ = 1 + 2 2 1 + 3 2 1 + 4 2 1 + 5 2 1 + 6 2 1 + 7 2 1 + ⋯ = ζ ( 2 ) = 6 π 2
Therefore, A = 6 1 , B = 2 , A B = 1 2