Alan says that two lines in the coordinate plane are perpendicular if and only if the slopes of the lines are m and 1 / m . Identify and correct two errors in Alan's statement. First, Alan should have said two (select) lines rather than two lines because (select) lines have undefined slope. Secondly, he should have had a (select) sign on one of his expressions for the slope because the slopes of perpendicular lines have a product of .