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AQA C4 Vectors need help??

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1. I dont understand how AQA vector questions work?? E.g. june 2015 question 6c, it gives you the length of DE/AC and i assumed you would need to work out the length using √a²+b²+c² but they worked it out using simple vectors

$\overrightarrow{OD} = \overrightarrow{OC} + \overrightarrow{AB}$

They have no mention of working out the length in the mark scheme. In other mark schemes when they have put length they expected you to work out the length of by using vectors e.g.
$\overrightarrow{OD} = \overrightarrow{OC} + \overrightarrow{AB}$
Can anyone explain to me how this works?
2. I did this paper yesterday but don't they say the alternative way is to use Pythagoras?
3. (Original post by Guls)
I did this paper yesterday but don't they say the alternative way is to use Pythagoras?
The alternative is for part b) not c) which is the one im talking about.
4. (Original post by fpmaniac)
I dont understand how AQA vector questions work?? E.g. june 2015 question 6c, it gives you the length of DE/AC and i assumed you would need to work out the length using √a²+b²+c² but they worked it out using simple vectors

$\overrightarrow{OD} = \overrightarrow{OC} + \overrightarrow{AB}$

They have no mention of working out the length in the mark scheme. In other mark schemes when they have put length they expected you to work out the length of by using vectors e.g.
$\overrightarrow{OD} = \overrightarrow{OC} + \overrightarrow{AB}$
Can anyone explain to me how this works?
You've made the question harder than it needs to be. You should draw a diagram and put all the relevant bits of information on it. You jus have to think about the geometry and what it's asking.

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