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( A ) as product of slopes of 2 perpendicular lines is -1.

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[Math Revolution GMAT math practice question]

Which of the following lines is perpendicular to \(y=(\frac{1}{2})x\)?

\(A. y=-2x\)
\(B. y=\frac{x}{2}\)
\(C. y=2x\)
\(D. y=\frac{-x}{2}\)
\(E. y=x\)

Two lines are perpendicular if, and only if, they satisfy one the two geometric configurations below:

1. One of them is vertical (slope not defined) and the other is horizontal (slope is zero)
2. Both are oblique (i.e., non-horizontal and non-vertical) and, in this case, the product of their slopes must be -1.

The line given in the question stem is oblique, with slope 1/2, therefore we look for a line with slope -2.

Answer: A

The above follows the notations and rationale taught in the GMATH method.
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=>

Two lines are perpendicular if and only if the product of their slopes is \(-1\).
Since \((\frac{1}{2})*(-2) = -1\), the slope of the line perpendicular to \(y = (\frac{1}{2})x\) is \(-2\).
The only answer choice with a slope of \(-2\) is A.

Therefore, A is the answer.
Answer: A
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