traingle question and simultaneous equations question Watch

Airess3
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Got these two questions wrong: Name:  20160308_010330.jpg
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Here's my working:[attach]5.1115751115951116e+23[/attach] Attachment 511157511159511200
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Zacken
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(Original post by Airess3)
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not sure what your working out for q8 is, but you need to take the dot product of the vectors (equate it to zero because perpendicular and form two simultaneous equations in a,b)
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Airess3
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(Original post by Zacken)
not sure what your working out for q8 is, but you need to take the dot product of the vectors (equate it to zero because perpendicular and form two simultaneous equations in a,b)
In my working, I already equated them to zero, I just moved the value of k over to the other side, hence the -1. I still don't know how to do it. And question 6.
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stochasym
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question 6
Let u=AB and v=AC
u=(1-2,-7-4,3-(-6))
then
Name:  ANGLE BETWEEN TWO VECTORS.JPG
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(to be continued.....by you)
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DylanJ42
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(Original post by Airess3)
x
for question 8 in your workings you are using i, j which isn't correct

After dot product only the coefficients of each component remain;

 \displaystyle \text{Let}\:A= a_1i + a_2j +a_3k \quad \text{and}\quad \displaystyle B = b_1i +b_2j +b_3k\:,

 \displaystyle A \cdot B = \begin{pmatrix}a_1 \\ a_2 \\ a_3 \end{pmatrix} \cdot \begin{pmatrix}b_1 \\ b_2 \\ b_3 \end{pmatrix} = a_1b_1 + a_2b_2 + a_3b_3

Notice how i, j, k are gone.

so  \displaystyle -3i + 2j = -1 should read  \displaystyle -3a +2b = -1

and  \displaystyle 2i - j = -1 should read  \displaystyle 2a - b = -1

which is just simultaneous equations, if you need help with those, here is an example, http://www.thestudentroom.co.uk/show...5&postcount=11

Edit: your mistake on Q8 was going from line 4 to line 5 of your workings, check that again
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Airess3
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(Original post by depymak)
question 6
Let u=AB and v=AC
u=(1-2,-7-4,3-(-6))
then
Name:  ANGLE BETWEEN TWO VECTORS.JPG
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Size:  15.3 KB
(to be continued.....by you)
I did that in the working posted above. I don't understand why it was wrong.
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stochasym
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sorry, I didn't see it.
(Original post by Airess3)
I did that in the working posted above. I don't understand why it was wrong.
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DylanJ42
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[QUOTE=Airess3;63263905]x[QUOTE]

Name:  vectors.jpg
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using  \displaystyle \overrightarrow{BA} is incorrect, you need to use  \displaystyle \overrightarrow{AB} instead, the direction does matter
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