Negative reciprocal (and the multiply-to-$-1$ check)
SAT glossary · Math
Definition
Perpendicular slopes satisfy m_1m_2=-1; parallel lines have equal slopes and different intercepts. Because three of four options on a perpendicular item are sign-and-flip permutations (-4/3 flips without negating, 3/4 negates without flipping, -3/4 is the parallel slope), the book insists the two-second product check is worth more than the memorized phrase.
Negative reciprocal (and the multiply-to-$-1$ check) in plain English
Two lines are perpendicular (meet at a perfect right angle) exactly when their slopes multiply together to equal -1 — written m_1 m_2=-1. That means each slope is the OTHER's 'negative reciprocal': flip the fraction upside down AND change its sign. Parallel lines, by contrast, just have the exact SAME slope (and different y-intercepts, or they'd be the same line). Test-makers love building wrong-answer choices that get exactly ONE of these two steps right: an option might flip the fraction but forget to negate it, or negate it but forget to flip it, or just be the parallel-line answer entirely. The safest defense is a two-second gut check: multiply your slope and the answer choice's slope together, and see if you actually get -1.
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