# [SOLVED]Angle between vector a and b

#### karush

##### Well-known member
$$\displaystyle a=-4i-2j$$ 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 $$\displaystyle \theta\approx72^o$$

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#### MarkFL

Staff member
Re: angle between vector a and b give answer to nearest degree.

Looks good to me!

#### karush

##### Well-known member
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
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
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
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
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.)