Question 299886
Which of the following statements are true ? 
a. the lines
y = 2x - 5, y = ax + 3
are perpendicular when a = -1/2
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The slope of the 1st eqn is +2.  The slope of any line perpendicular is the negative inverse, = -1/2.
So it's true.
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b. the lines
y = 4x + 3, y = ax + 2
are parallel when a = 4
The slope of the 1st eqn is +4.  All lines parallel will have the same slope, so it's true.
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c. a vertical line has undefined slope. 
True.  Parallel to the y-axis is often referred to as vertical.
Slope = change in y/change in x
The change in x = 0 --> undefined.