Not So Tedious Multiplication

Algebra Level 1

1 2 × 2 3 × 3 4 × × 3442 3443 = ? \large \dfrac{1}{2} \times \dfrac{2}{3}\times \dfrac{3}{4}\times\cdots\times \dfrac{3442}{3443} = \, ?

1 3441 \frac{1}{3441} 1 3442 \frac{1}{3442} 1 3443 \frac{1}{3443}

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2 solutions

Chew-Seong Cheong
Oct 23, 2016

Relevant wiki: Telescoping Series - Product

We can cancel out the numbers from the numerator of a fraction and a denominator from another fraction in pairs:

P = 1 2 2 3 3 4 3441 3442 3442 3443 = 1 2 2 3 3 4 3441 3442 3442 3443 = 1 3443 \begin{aligned} P & = \frac 12 \cdot \frac 23 \cdot \frac 34 \cdots \frac {3441}{3442} \cdot \frac {3442}{3443} = \frac 1{\cancel{2}} \cdot \frac {\cancel{2}}{\cancel{3}} \cdot \frac {\cancel{3}}{\cancel{4}} \cdots \frac {\cancel{3441}}{\cancel{3442}} \cdot \frac {\cancel{3442}}{3443} = \boxed{\dfrac 1{3443}} \end{aligned}

consider the first three terms

1 2 \frac{1}{2} * 2 3 \frac{2}{3} * 3 4 \frac{3}{4} = = 1 4 \frac{1}{4}

2 and 2 cancels out and 3 and 3 cancels out

so the answer to the problem is 1 3443 \frac{1}{3443}

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