MHB What is the angle between vector a and b if a=-4i-2j and b=i-7j?

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The discussion centers on calculating the angle between two vectors, a and b, defined as a = -4i - 2j and b = i - 7j. The dot product is calculated as a·b = 10, and the magnitudes of the vectors are ||a|| = 2√5 and ||b|| = 5√2. Using these values, the cosine of the angle θ is determined to be approximately 72 degrees. Some participants express confusion over the use of different variable names (u and d) instead of a and b, suggesting it may be a typographical error. The conversation concludes with an acknowledgment of the error and a light-hearted comment about editing posts.
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$$a=-4i-2j$$ and $$b=i-7j$$

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

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

$$\cos\theta = \frac{10}{10\sqrt{10}}$$ so $$\theta\approx72^o$$
 
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Re: angle between vector a and b give answer to nearest degree.

Looks good to me! (Sun)
 
Re: angle between vector a and b give answer to nearest degree.

Thnx to MHB I make some progress...
 
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.
 
Re: angle between vector a and b give answer to nearest degree.

HallsofIvy said:
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.
 
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.
 
Re: angle between vector a and b give answer to nearest degree.

Prove It said:
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.)
 
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