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# C4 vector question - I know I'm being an idiot... watch

1. ...but I just can't figure out what the actual method is.

The question is:

a = 5i + 4j
b = 2i + 3j

Find a.b

I know that:

a.b = |a||b|cosx

So what I tried was drawing out vectors a and b, working out the distance between the end of the point to make a triangle, and then using the cosine rule to work out a value for cosx.

The answer is 22, but I get 36. I've done the qusetion twice in this way and get 36 -so can someone show me how to do this?

Cheers.
2. There's another expression for the dot product of two vectors. Namely, . You're making a lot of extra work for yourself if you try and find the angle geometrically (unless it's really obvious what the angle is; but it's not here).
3. (Original post by TenaciousMonkey)
...but I just can't figure out what the actual method is.

The question is:

a = 5i + 4j
b = 2i + 3j

Find a.b

I know that:

a.b = |a||b|cosx

So what I tried was drawing out vectors a and b, working out the distance between the end of the point to make a triangle, and then using the cosine rule to work out a value for cosx.

The answer is 22, but I get 36. I've done the qusetion twice in this way and get 36 -so can someone show me how to do this?

Cheers.
use Cosx = (a.b)/(|a|x|b|)

so in your case it would be...

(5x2) + (4x3) = 22

then do 22 divided by...

|a| x |b|

this gives you cosx.. do cos^-1 of your answer to get x
4. (Original post by cor_fortis)
use Cosx = (a.b)/(|a|x|b|)

so in your case it would be...

(5x2) + (4x3) = 22

then do 22 divided by...

|a| x |b|

this gives you cosx.. do cos^-1 of your answer to get x
The OP was trying to find a.b, not the angle (trying to find the angle was their roundabout way of finding a.b).
5. (Original post by nuodai)
The OP was trying to find a.b, not the angle (trying to find the angle was their roundabout way of finding a.b).
i just told him the a.b method. i'm pretty sure he's smart enough to realise which section he needs.
6. Multiply rows and add !
7. (Original post by cor_fortis)
i just told him the a.b method. i'm pretty sure he's smart enough to realise which section he needs.
No.

I don't think OP knows the "direct" way of finding the dot product of two vectors like nuodai just mentioned.
8. (Original post by soutioirsim)
No.

I don't think OP knows the "direct" way of finding the dot product of two vectors like nuodai just mentioned.
Bit harsh on the OP
9. (Original post by cor_fortis)
Bit harsh on the OP
I was saying "no" to you.
10. Nuodai has it correct, you're kind of complicating it OP!
11. Thanks to everyone for replying.

Like I said... I was being pretty dumb, there. My intelligence is definitely questionable

Thanks for helping!

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