SAT Math Multiple Choice Question 786: Answer and Explanation
6. Line L has an undefined slope. Line M is perpendicular to line L. Which of the following could be the equation of line M?
Correct Answer: B
Category: Heart of Algebra / Linear Equations
Strategic Advice: A line with an undefined slope is a vertical line, or in other words, parallel to the y-axis.
Getting to the Answer: A line that is perpendicular to a vertical line must be a horizontal line, which has a slope of 0. Its equation, therefore, will be of the form y = 0x + b, or just y = b, where b is a constant. The only equation that meets this criterion is choice (B).