Two lines are perpendicular. What will the product of their slopes be?
Here’s what we know. The slope of perpendicular lines are the negative reciprocal of one another. The reciprocal is also called the multiplicative inverse. And the multiplicative inverse is what you multiply by a number to equal one.
So two reciprocals multiplied together equal one. So when we multiply any slope by its reciprocal, it multiplies together to equal one.
But we’re dealing with not just reciprocal slopes, but negative reciprocal slopes. Perpendicular lines are the negative reciprocal, which means when you multiply a slope by a perpendicular slope, they multiply together to equal negative one. For any two lines that are perpendicular, if you multiply their slopes together, they will equal negative one.