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Two straight lines have rates of change of 5 and minus 0.2. Multiplying the two rates together gives minus 1. What does that tell you about the lines?
AThey both pass through the same start
BThey cross each other at a right angle
CThey are the very same line drawn twice
DThey are parallel and never touch at all
Answer & Solution
Correct answer: B. They cross each other at a right angle
1. Two rates of change that multiply to minus 1 are negative reciprocals of each other.
2. Lines with negative reciprocal rates meet at a right angle.
3. Parallel lines instead share the same rate, so option D would need both rates to be 5.
4. Nothing here says where either line starts, so option A cannot be judged from the rates alone.
5. So the lines cross at a right angle, option B.
_Source: OpenStax College Algebra (CC BY 4.0), Ch 4 "Linear Functions", section 4.1 Linear Functions_
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