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I recently got this question in exam. This seems like a very basic question. But since I made it wrong , It might be my basics are not strong.

enter image description here

I solved it this way.

1/1 / 25/1

<p>1*1/1*25</p>

<p>1/25</p>

I got answer as 1/25. But the answer is 25.

How ?Can somebody explain.

Martin Sleziak
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vikiiii
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  • An expression like $a/b/c/d$ is ambiguous. It could mean $(a/b)/(c/d)$, or $a/(b/(c/d))$, or a few other things. It would be a good idea to put in a few parentheses so what you write is not ambiguous. The original question is ambiguous, for similar reasons. Is it $(1/1)/(1/5^{-2})$? $1/(1/(1/5^{-2}))$? – Gerry Myerson Sep 15 '13 at 10:36
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    If this is the picture from your exam then I think you are right because of the length of the central line and since $$\frac{\frac 11}{\frac 1{5^{-2}}}=\frac 11 \frac {5^{-2}}1=5^{-2}$$ – Raymond Manzoni Sep 15 '13 at 10:37

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Denote $\frac{1}{x}$ by $x^{-1}$. By basic property of the exponential, if $n, m \in \mathbb{Z}$, then $(x^{m})^{n} = x^{mn}$.

Your expression above is $(((5^{-2})^{-1})^{-1})^{-1} = 5^{(-2)(-1)(-1)(-1)} = 5^2 = 25$.

William
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  • can you explain in a simpler way? – vikiiii Sep 15 '13 at 10:31
  • If this is true, it's just an error in the problem – J.H. Sep 15 '13 at 10:34
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    The answer is $25$ if the parenthesis is $(1 / (1 / (1/ 5^{-2})))$. Taking a more careful look at the picture, it seems that it could be interpreted as $(1/1)/(1/5^{-2}) = 1 / 25$. I think the picture may be ambiguous. – William Sep 15 '13 at 10:42
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    If we assume the OP wrote the division lines properly, the expression is $,\frac{\frac11}{\frac1{5^{-2}}}=5^{-2}=\frac1{25};$ and the OP's right. – DonAntonio Sep 15 '13 at 11:31