1997 AMC 8 Problem 19

Attempt Problem 19 of the 1997 AMC 8 below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 1997 AMC 8 solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

19.

If the product 32⋅43⋅54⋅65⋅…⋅ab=9,\dfrac{3}{2}\cdot \dfrac{4}{3}\cdot \dfrac{5}{4}\cdot \dfrac{6}{5}\cdot \ldots\cdot \dfrac{a}{b} = 9, what is the sum of aa and b?b?
1111
1313
1717
3535
3737
Answer: D
Concepts:telescopingfraction
Small hint:
After cancellation, only the first denominator and last numerator remain
Big hint:
The product telescopes
Solution:
Note that the numerator of each fraction cancels with the denominator of the fraction to its right.
We can then cancel out all these terms to get a final equation of a2=9⇒a=18⇒b=17. \dfrac{a}{2} = 9 \Rightarrow a = 18 \Rightarrow b = 17.
The desired sum is then 18+17=35. 18 + 17 = 35.
Thus, D is the correct answer.
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