Welcome to our community

Be a part of something great, join today!

[SOLVED] Angle between vector a and b

karush

Well-known member
Jan 31, 2012
2,716
\(\displaystyle a=-4i-2j\) and \(\displaystyle b=i-7j\)

\(\displaystyle a\cdot b=(-4)(1)+(-2)(-7)=10\)
\(\displaystyle ||a||=\sqrt{(-4)^2+(-2)^2}=2\sqrt{5}\)

\(\displaystyle ||b||=\sqrt{1^2+(-7)^2}=5\sqrt{2} \)

\(\displaystyle \cos\theta = \frac{10}{10\sqrt{10}}\) so \(\displaystyle \theta\approx72^o\)
 
Last edited:

MarkFL

Administrator
Staff member
Feb 24, 2012
13,775
Re: angle between vector a and b give answer to nearest degree.

Looks good to me! (Sun)
 

karush

Well-known member
Jan 31, 2012
2,716
Re: angle between vector a and b give answer to nearest degree.

Thnx to MHB I make some progress.....
 

HallsofIvy

Well-known member
MHB Math Helper
Jan 29, 2012
1,151
Re: angle between vector a and b give answer to nearest degree.

Frankly, it doesn't make much sense to me. You give two complex numbers, a and b, and then immediately take the dot product of two vectors, u and d. Where are u and d defined?
I presume that you are taking u= a and v= b but I don't see why you changed their names.
 

karush

Well-known member
Jan 31, 2012
2,716
Re: angle between vector a and b give answer to nearest degree.

Frankly, it doesn't make much sense to me. You give two complex numbers, a and b, and then immediately take the dot product of two vectors, u and d. Where are u and d defined?
I presume that you are taking u= a and v= b but I don't see why you changed their names.
You're correct, the original problem just uses $a$ and $b$; $u$ and $d$ should not be used.
 

Prove It

Well-known member
MHB Math Helper
Jan 26, 2012
1,403
Re: angle between vector a and b give answer to nearest degree.

I expect it's just a typo, and u is supposed to be a and d is supposed to be b.
 

HallsofIvy

Well-known member
MHB Math Helper
Jan 29, 2012
1,151
Re: angle between vector a and b give answer to nearest degree.

I expect it's just a typo, and u is supposed to be a and d is supposed to be b.
Hey, no fair editing after my post- that makes me look like a fool! (And I was hoping I could keep that quiet.)