# A-LEVEL MATHS : VECTORS help

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Thread starter 1 year ago
#1
I'm really stuck on this question:

Relative to a fixed origin, point A has position vector 6i-3j and point B has position vector 4i+2j. Find the magnitude of the vector AB and the angle it makes with unit vector i.
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1 year ago
#2
(Original post by May1200)
I'm really stuck on this question:

Relative to a fixed origin, point A has position vector 6i-3j and point B has position vector 4i+2j. Find the magnitude of the vector AB and the angle it makes with unit vector i.
to find AB you need to do B - A and then you will get the magnitude of the vector... can't remember how to do the angle srry
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1 year ago
#3
to find angle it makes with i, you can use this nifty formula :

cos theta = coefficient of i / maginitude of AB

so in this case:

theta = arc cos (-2 / magnitude of AB)
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1 year ago
#4
so for this question you need to know what the magnitude is and dot product
answer down

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1 year ago
#5
(Original post by uraverageperson)
so for this question you need to know what the magnitude is and dot product
answer down

Dot product is unneccessary for this question as its a technique that shouldn't be required for an A-level maths question, rather a FM question.
My method is a more efficient, A-level maths method
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1 year ago
#6
(Original post by May1200)
I'm really stuck on this question:

Relative to a fixed origin, point A has position vector 6i-3j and point B has position vector 4i+2j. Find the magnitude of the vector AB and the angle it makes with unit vector i.
so first calculate the magnitude of vector AB. This will be root 29. The magnitude of unit vector i is 1. Calculate the dot product of AB and i. This will be -2. So cos theta= -2/root 29
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1 year ago
#7
mb thats all i could think of at the time
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