1. If two nonvertical lines are parallel, what do we know about their slopes? If two lines are perpendicular and neither is parallel to an axis, what do we know about their slopes? Why must the lines be nonvertical?

Two straight lines with slopes m and m' are parallel when m = m'

Two straight lines with slopes m and m' are perpendicular when m * m' = - 1.

Step-by-step explanation:

Let us assume that the two non-vertical lines in the slope-intercept form are

y = mx + c ... (1) and

y = m'x + c' ... (2)

If those two lines are parallel then we can say the slope of them will be the same i. e. m = m'

Now, if given two straight lines (1) and (2) are perpendicular to each other and neither of them is parallel to the axes, then we can write m * m' = - 1. (Answer)