## anonymous 5 years ago If vector a=-8(vector b) and vector c=7(vector b) and what is angle between a and c ??

1. amistre64

cos(t) = a*c/|a||c|

2. amistre64

b*b = |b|^2

3. amistre64

<-8xb, -8yb> < 7xb, 7yb> -------------- -xb + -yb = -1<b>

4. amistre64

thats messed up lol

5. anonymous

yeah.. i didnt really understand that..

6. anonymous

what do u mean in ur 3rd reply..??

7. amistre64

a*c = -8<xb,yb> * 7<xb,yb>

8. amistre64

-56x^2b + -56 y^b is the only values we can get from it right?

9. amistre64

-56|b|^2 is the top value

10. amistre64

the bottom is -8|b| * 7|b| = -56|b|^2 cos(t) = 1

11. amistre64

t = 90 begrees right?

12. amistre64

so hard to type in the dark lol

13. amistre64

or it might just be 180 degrees; since a and c are scalars of b and they are facing inopposite directions.

14. anonymous

I worked it similarly. I don't know if we got same place. $-8b.-7b =\left| a \right|\left| c \right|\cos \theta$ $-8b.-7b =8b7b \cos \theta$ $-1=\cos \theta$ $\cos^{-1} -1=\theta$ $\theta = 3.14 (radians)$

15. anonymous

I didn't dot the left hand side. May be that is a mistake that needs to be cleaned up. I should go to sleep now.