# Perpendicular lines, equal aspect

#### apple2357

##### Junior Member
Something i had never fully appreciated, until recently is when you talk about two lines being perpendicular the scales are automatically assumed to be equal aspect , otherwise the lines do not 'look' perpendicular? Is this always clear to students? Otherwise you get some odd looking results?

Is y=2x perpendicular to y= -0.5x ? Or only when the scales are equal aspect?

#### MarkFL

##### Super Moderator
Staff member
Two lines in the plane are perpendicular when the product of their slopes is -1. In order for them to look perpendicular when graphed, yes, the two axes must be equally scaled.

#### Dr.Peterson

##### Elite Member
To put it another way, when we talk about perpendicular lines on a coordinate plane (or about angles in general), we are assuming that the axes have the same scale.

#### apple2357

##### Junior Member
yes, it appears so. Not something i had ever thought about before!

#### lookagain

##### Senior Member
Two lines in the plane are perpendicular when the product of their slopes is -1. In order for them to look perpendicular when graphed, yes, the two axes must be equally scaled.
Also, two lines in the plane are perpendicular if the slope of one of them is zero and the slope of the other is undefined.