Finding perpendicular lines without graphing

[cokefree]

Could some tell me what's the other way you could find if lines are perpendicular, other than graphing. I know another way to do it, and its algebracaly, but there's another way you could do it algebracaly too.

 

[himanshu121]

Find the slope for the two lines
Their product should be equal to -1 if the lines are perpendicular

 

[selfAdjoint]

Form a unit vector along each line, with components. Then do an inner product of the vectors to see if it's zero. Same work in a different dress.