c4 vectors

hello. need help with part 2 of this question, i thought you would dot product the two vectors, divide it by the product of their respective magnitude and then inverse cos the hwole thing. i don not get the right answer and am not sure what else to do

$\cos \theta = \frac{a{\cdot}b}{|a|{\cdot}|b|}$

What's the question?

what question?

hello. need help with part 2 of this question, i thought you would dot product the two vectors, divide it by the product of their respective magnitude and then inverse cos the hwole thing. i don not get the right answer and am not sure what else to do

ie,i was using this formula
$\cos \theta = \frac{a{\cdot}b}{|a|{\cdot}|b|}$

and got

$\cos \theta = \frac{-21}{\sqrt{29}{\cdot}3\sqrt{10}}$

Your magnitudes are incorrect.
Edit: i realise you got the correct ones, your notation confused me

haha! can you identify where else i may have gone wrong then? or am i approaching the question in the wrong way?

To find the acute angle from the obtuse angle - you simply use 180 or pi - (STUFF FROM YOUR CALCULATOR)

To find the acute angle from the obtuse angle - you simply use 180 or pi - (STUFF FROM YOUR CALCULATOR)

Sorry to intrude but wouldn't that still give you the wrong answer because the question wanted the angle OAB so wouldn't you have to use the result from the first part to help work out your angle?
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strange. i still seem to get the wrong answer. the mark scheme states the answer is 43 to one degree, or 0.75 in radians.

my answer when doing this gives me 65.7 degrees

Apparently so. . .
Given that you know all three lengths of the sides of the triangle I wouldn't bother using a dot product. But it seems to want you to.

the question states i must use a scalar product! plus my other trigonmetry is a bit rusty hahah

the question states i must use a scalar product! plus my other trigonmetry is a bit rusty hahah

Cos rule never hurt anyone
Though it seems to be giving the same result as the dot product.

You guys need to read the question a bit more carefully

You are asked for angle OAB...the dot product you are working out is relevant to angle AOB.

You will find that the work you've done finding the length of vector AB will help you out