AlgebraHardSlope of a perpendicular line
Algebra practice question
In the xy-plane, the line y = mx + 4 is perpendicular to the line y = 2x − 1. What is the value of m?
Answer: -1/2 or -0.5 or −1/2 or −0.5
Perpendicular lines have slopes whose product is −1. The second line's slope is 2, so m = −1/2.
Common wrong answers
- If you answered 2
- 2 is the slope of a PARALLEL line.
- If you answered -2
- −2 has the right sign but isn't the reciprocal; 2 × (−2) = −4, not −1.
- If you answered 0.5
- 1/2 is the reciprocal without the sign change; 2 × 1/2 = 1, not −1.
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