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Core 4 Vectors Exam Question

Hi,

I'm having some issues solving 7c. I already have AP, which I know is [6, 12, -18], but I can't work out how to get AB.

From the mark scheme, it says AP^(2) = 504, which I understand, but then it says 2AP^(2) = AB^(2) and this is the part I don't understand. How do I get 2AP^(2) = AB^(2).

Any help would be much appreciated. Apologies for the poor quality photo btw.
Reply 1
perp.png
Looks like a right angle to me. There's a reason they asked you to show 1 \ell _1 and 2 \ell _2 are perpendicular.

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