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Algebra Difficulty 2.3 Junior Find the answer

Suppose that 12×23×34×45××n1n=18\sqrt{\frac{1}{2} \times \frac{2}{3} \times \frac{3}{4} \times \frac{4}{5} \times \cdots \times \frac{n-1}{n}} = \frac{1}{8}. What is the value of nn?

A number or a short expression. Spacing and $ signs are ignored.

Solution

Since 12×23×34×45××n1n=18\sqrt{\frac{1}{2} \times \frac{2}{3} \times \frac{3}{4} \times \frac{4}{5} \times \cdots \times \frac{n-1}{n}} = \frac{1}{8}, then squaring both sides, we obtain 12×23×34×45××n1n=164\frac{1}{2} \times \frac{2}{3} \times \frac{3}{4} \times \frac{4}{5} \times \cdots \times \frac{n-1}{n} = \frac{1}{64}. Simplifying the left side, we obtain 1×2×3×4××(n1)2×3×4×5××n=164\frac{1 \times 2 \times 3 \times 4 \times \cdots \times (n-1)}{2 \times 3 \times 4 \times 5 \times \cdots \times n} = \frac{1}{64} or 1×(2×3×4××(n1))(2×3×4××(n1))×n=164\frac{1 \times (2 \times 3 \times 4 \times \cdots \times (n-1))}{(2 \times 3 \times 4 \times \cdots \times (n-1)) \times n} = \frac{1}{64} and so 1n=164\frac{1}{n} = \frac{1}{64} which means that n=64n = 64.

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