Question

Is it possible for two lines with negative slopes to be perpendicular? Justify your answer.

Answer 1

No

Explanation:

The slopes of perpendicular lines must always be the negative reciprocal of each other. If one slope is negative, for the other to be perpendicular to it, it must be positive, and vise versa.

Answer 2

No

Explanation:

Interesting question. Subtle.

The formula for two lines to be perpendicular is m1 * m1 = -1 That means that two lines cannot be minus -- only 1 of them can be. The reason is that if both were minus when multiplied together would give a positive result -- not minus one. For example, suppose you have

y = x + 2

y = 2x + 3.

These 2 lines will cross and they are not parallel, but neither are they perpendicular. I'm not going to solve this to find out where they cross -- that's not what you are asking.

Now suppose you have two other lines

y = - 2x + 3

y = 1/2 x + 1

These two lines should be perpendicular

-2 * 1/2 = - 1 which is the condition of perpendicular.

Separate graphs have been uploaded to help you understand.