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June 3rd, 2017, 12:10 PM   #1
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I need a little help with vectors.

Okay so, P is bisecting ED and R is bisecting FE. This is my attempt but the answer in the books says it's not true. Can somebody tell me what I do wrong? Thank you!
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 June 3rd, 2017, 12:16 PM #2 Senior Member     Joined: Sep 2015 From: Southern California, USA Posts: 1,412 Thanks: 716 what are you trying to do?
June 3rd, 2017, 12:33 PM   #3
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Quote:
 Originally Posted by romsek what are you trying to do?
Оh, sorry I forgot to say that. I need to express vector AS in terms of vectors a and b

 June 3rd, 2017, 02:28 PM #4 Global Moderator   Joined: Dec 2006 Posts: 17,919 Thanks: 1386 I got -(3/13)a + (12/13)b, but I haven't checked your attempt yet.
June 4th, 2017, 01:08 AM   #5
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Quote:
 Originally Posted by skipjack I got -(3/13)a + (12/13)b, but I haven't checked your attempt yet.
That is exactly the answer in the book. Can you please check my answer or write yours here. It will mean a world to me

 June 4th, 2017, 02:06 AM #6 Global Moderator   Joined: Dec 2006 Posts: 17,919 Thanks: 1386 I assume the hexagon is regular. With A as origin, vector AS is a fraction, m, of vector AP, i.e. m(-(1/2)a + 2b). Similarly, vector RS is a fraction, n, of vector RB, i.e. n(2a - (3/2)b), so vector AS = AR + RS = -a + (3/2)b + n(2a - (3/2)b). As these two expressions for vector AS are equivalent, -m/2 = 2n - 1 and 2m = 3/2 - (3/2)n, so m = 6/13 and n = 5/13. Substituting these values into either expression for vector AS gives -(3/13)a + (12/13)b. Your method was along much the same lines, but you found vector BR incorrectly.
June 4th, 2017, 04:06 AM   #7
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Quote:
 Originally Posted by skipjack I assume the hexagon is regular. With A as origin, vector AS is a fraction, m, of vector AP, i.e. m(-(1/2)a + 2b). Similarly, vector RS is a fraction, n, of vector RB, i.e. n(2a - (3/2)b), so vector AS = AR + RS = -a + (3/2)b + n(2a - (3/2)b). As these two expressions for vector AS are equivalent, -m/2 = 2n - 1 and 2m = 3/2 - (3/2)n, so m = 6/13 and n = 5/13. Substituting these values into either expression for vector AS gives -(3/13)a + (12/13)b. Your method was along much the same lines, but you found vector BR incorrectly.
Okay, now I understand it. Thank you veryyy much

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