2005 AMC 12B Problems/Problem 17

Problem

How many distinct four-tuples $(a,b,c,d)$ of rational numbers are there with

$a\cdot\log_{10}2+b\cdot\log_{10}3+c\cdot\log_{10}5+d\cdot\log_{10}7=2005?$

$\mathrm{(A)}\ 0 \qquad \mathrm{(B)}\ 1 \qquad \mathrm{(C)}\ 17 \qquad \mathrm{(D)}\ 2004 \qquad \mathrm{(E)}\ \text{infinitely many}$

Solution

Using the laws of logarithms, the given equation becomes

$\log_{10}2^{a}+\log_{10}3^{b}+\log_{10}5^{c}+\log_{10}7^{d}=2005$ $\Rightarrow \log_{10}{2^{a}\cdot 3^{b}\cdot 5^{c}\cdot 7^{d}}=2005$ $\Rightarrow 2^{a}\cdot 3^{b}\cdot 5^{c}\cdot 7^{d} = 10^{2005}$

As $a,b,c,d$ must all be rational, and there are no powers of $3$ or $7$ in $10^{2005}$ , $b=d=0$ . Then $2^{a}\cdot 5^{c}=2^{2005}\cdot 5^{2005} \Rightarrow a=c=2005$ .

Only the four-tuple $(2005,0,2005,0)$ satisfies the equation.

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2005 AMC 12B Problems/Problem 17

Problem

Solution

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