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2002 AMC 10B Problem 12

Problem 12 of 25IntermediateAlgebra

For which of the following values of kk does the equation x−1x−2=x−kx−6\dfrac{x - 1}{x - 2} = \dfrac{x - k}{x - 6} have no solution for x?x?

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Solution

Cross multiplying gives (x−1)(x−6)=(x−2)(x−k),(x - 1)(x - 6) = (x - 2)(x - k), which expands to x2−7x+6=x2−(2+k)x+2k. \begin{aligned} x^2 - 7x + 6 &= x^2 - (2 + k)x \\ &\quad {}+ 2k. \end{aligned} Cancelling x2x^2 leaves (k−5)x=2k−6.(k - 5)x = 2k - 6. For kk equal to 1,1, 2,2, 3,3, or 4,4, this gives a valid value of xx that is neither excluded denominator value 22 nor 6.6. For k=5,k=5, the equation instead becomes 0⋅x=4,0\cdot x=4, which has no solution. Thus, the correct answer is E.
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