Question 66590
Determine which two equations represent perpendicular lines.  
(a) y=5x-5  (b) y=1/5x+5  (c) y=-1/5x+5  (d) y=1/5x-5
:
(a) and (c) are perpendicular, this is how you can tell:
These lines are all in slope intercept form, {{{highlight(y=mx+b)}}}, where m=slope and b=y-intercept.
Perpendicular lines have slopes that are negative reciprocals of each other, that means their slopes have opposite signs and are upside-down.  Because of that if you multiply their slopes you get -1
The slope of (a) is m=5, the slope of (c) is m=-1/5.  They are negative reciprocals of each other, {{{5*(-1/5)=-5/5=-1}}}
Happy Calculating!!!