1. May 21, 2012

### physics kiddy

To add two vectors, a$\rightarrow$ and b$\rightarrow$ we use the formula $\sqrt{}$a2+b2+2abcosθ

where θ is the smallest angle between vectors a and b. Is there any way I can use the bigger angle to calculate the sum ? It's not a homework, just a question from my intution.

I have attached a pic also... Thanks in advance for help

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2. May 21, 2012

### tiny-tim

hi physics kiddy!
you mean 360° - θ ?

but cos(360° - θ) equals cosθ

btw, the formula is easier to understand if you draw a and b head-to-tail (instead of tail-to-tail)

3. May 21, 2012

### Infinitum

To use the bigger angle, all you need to do is to express cos(360-x) in terms of cos(x), which in fact is cos(360-x) = cos(x)