Are Perpendicular Lines Always Right Angles, Even if They Appear Acute?

[RuroumiKenshin]

The other day, I was looking at two intersecting straight lines. They were drawn from a certain angle so that they looked acute. But if you sort of changed the angle of your view (by turning the paper a bit to the side), it looked like, quite simply, a coordinate plane without a graph. So, since the two lines where perpendicular to each other, does it mean they are right angles? Even if they only look acute?
My sister and I had quite a bit of a quarrel over this...and I wanted an answer that was more substantiated, or rather "standard" so that more people could agree. Okay, so bring it on...

 

[HallsofIvy]

HallsofIvy

The other day, I was looking at two intersecting straight lines. They were drawn from a certain angle so that they looked acute. But if you sort of changed the angle of your view (by turning the paper a bit to the side), it looked like, quite simply, a coordinate plane without a graph. So, since the two lines where perpendicular to each other, does it mean they are right angles? Even if they only look acute?/QUOTE]

I'm not sure I understand your question. Certainly if two lines ARE perpendicular, then they cross at right angles. (On the other hand, no, "they" are NOT right angles because lines are not angles!)

How do you KNOW they are perpendicular if they LOOK acute?

 

[RuroumiKenshin]

uhh...that was my question.
By definition, perpendicular angles are right angles right?

or is the whole situation relative? (i personally don't believe so)

 

[Ed Quanta]

I don't think I follow what you say completely. Angle is a relative quantity due to Lorentz Transformations, and the way space changes while traveling at different speeds. But yeah, I don't think that was quite what you were asking.

 

[HallsofIvy]

Oh, for God's sake don't bring relativity into it!


Yes, "perpendicular" means "at right angles".

Two lines are perpendicular if and only if they cross at right angles.