Difference between revisions of "2005 AMC 10B Problems/Problem 17"

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2^3&=2^{2\cdot a\cdot b\cdot c\cdot d}\\
 
2^3&=2^{2\cdot a\cdot b\cdot c\cdot d}\\
 
3&=2\cdot a\cdot b\cdot c\cdot d\\
 
3&=2\cdot a\cdot b\cdot c\cdot d\\
a\cdot b\cdot c\cdot d&=\boxed{\textbf{(B) }\ \dfrac{3}{2}}\\
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a\cdot b\cdot c\cdot d&=\boxed{\textbf{(B) }\dfrac{3}{2}}\\
 
\end{align*}</cmath>
 
\end{align*}</cmath>
  

Revision as of 12:56, 16 December 2021

Problem

Suppose that $4^a = 5$, $5^b = 6$, $6^c = 7$, and $7^d = 8$. What is $a \cdot b\cdot c \cdot d$?

$\textbf{(A) } 1 \qquad \textbf{(B) } \frac{3}{2} \qquad \textbf{(C) } 2 \qquad \textbf{(D) } \frac{5}{2} \qquad \textbf{(E) } 3$

Solution

\begin{align*} 8&=7^d \\ 8&=\left(6^c\right)^d\\ 8&=\left(\left(5^b\right)^c\right)^d\\ 8&=\left(\left(\left(4^a\right)^b\right)^c\right)^d\\ 8&=4^{a\cdot b\cdot c\cdot d}\\ 2^3&=2^{2\cdot a\cdot b\cdot c\cdot d}\\ 3&=2\cdot a\cdot b\cdot c\cdot d\\ a\cdot b\cdot c\cdot d&=\boxed{\textbf{(B) }\dfrac{3}{2}}\\ \end{align*}

Solution using logarithms

We can write $a$ as $\log_4 5$, $b$ as $\log_5 6$, $c$ as $\log_6 7$, and $d$ as $\log_7 8$. We know that $\log_b a$ can be rewritten as $\frac{\log a}{\log b}$, so $a*b*c*d=$ \[\frac{\log5}{\log4}\cdot\frac{\log6}{\log5}\cdot\frac{\log7}{\log6}\cdot\frac{\log8}{\log7}=\]

\[\frac{\log8}{\log4}=\]

\[\frac{3\log2}{2\log2}=\]

\[\boxed{\frac{3}{2}}\]

Solution using logarithm chain rule

As in solution 2, we can write $a$ as $\log_4 5$, $b$ as $\log_56$, $c$ as $\log_67$, and $d$ as $\log_78$. $a*b*c*d$ is equivalent to $(\log_4 5)*(\log_5 6)*(\log_6 7)*(\log_7 8)$. Note that by the logarithm chain rule, this is equivalent to $\log_4 8$, which evaluates to $\frac{3}{2}$, so $\boxed{B}$ is the answer. ~solver1104

See Also

2005 AMC 10B (ProblemsAnswer KeyResources)
Preceded by
Problem 16
Followed by
Problem 18
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All AMC 10 Problems and Solutions

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