# Angle between two vectors

• Nov 18th 2010, 08:32 AM
Chokfull
Angle between two vectors
Im trying to find the angle between two vectors on Maple 13. I type in: "VectorAngle((3,7,7),(8,6,2))" and it gives me $arccos(\frac{20}{1391}\sqrt{107}\sqrt{26})$. This simplifies to .71 radians or about $40^o$. The problem is the angle is supposed to be over 70 degrees from what I've worked out. Are my calculations wrong or did i type something in wrong in the calculator?

I'm not sure if this is important, but when i typed in my vectors I, instead of the inside parentheses in red above, used some strange symbols that looked like < and > but taller and skinnier, but i don't know how to type these i had just copy-pasted them from somewhere else. When i copy-paste them here, though, they show up as <>, but they don't look the same when i try to type them into Maple as <>. I just don't get this at all :p
• Nov 18th 2010, 08:47 AM
Plato
Mathcad gave me the exact same result.
• Nov 18th 2010, 10:20 AM
Ackbeet
There are ambiguities in the inverse cosine function. Another answer in the range of 0 to 360 degrees is 360 - 40 = 320 degrees. Does that answer make sense?
• Nov 22nd 2010, 08:10 AM
Chokfull
Yeah-- $\cos40^o=\cos 320^o$ is what you're saying, right? Either way, it's not the answer I'm looking for. The answer I want is about 70 degrees.
• Nov 22nd 2010, 08:14 AM
Chokfull
Quote:

Originally Posted by Plato
Mathcad gave me the exact same result.

Mkay, thanks. I figured my calculations were probably what went wrong, but I wanted to ask here first. My vectors are right, and I'm fairly certain my 70 degrees is right, but i guess it's back to the drawing board. :P
• Nov 22nd 2010, 08:16 AM
Plato
Quote:

Originally Posted by Chokfull
Yeah-- $\cos40^o=\cos 320^o$ is what you're saying, right? Either way, it's not the answer I'm looking for. The answer I want is about 70 degrees.

You may well be looking for an angle of $70^o$ but with those two vectors your not going to find it.
Again, check the vectors. Did you copy them correctly?
Or could there be a different way of reading the problem?
• Nov 22nd 2010, 08:40 AM
Chokfull
Yeah, like i said, I'm going to crunch all my numbers again. I'll probably post again if I don't find any mistakes in my math, but I'll betcha money I am.