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    Why is this wrong... ??
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    (Original post by pippabethan)
    Why is this wrong... ??
    It's not wrong, you've just found the obtuse angle between the two lines, the question wants the acute angle.
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    (Original post by Zacken)
    It's not wrong, you've just found the obtuse angle between the two lines, the question wants the acute angle.
    Now that was a quick response.
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    (Original post by Zacken)
    It's not wrong, you've just found the obtuse angle between the two lines, the question wants the acute angle.
    Okay, thank you! I was worried I didn't understand it properly! How do I 'translate' this into the angle they want?
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    (Original post by pippabethan)
    Okay, thank you! I was worried I didn't understand it properly! How do I 'translate' this into the angle they want?
    Well, you have \cos x = -\frac{7}{9} and you know x is obtuse, so you really want \cos (\pi - x) = -\cos x = - (-7/9) = 7/9
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    (Original post by Zacken)
    Well, you have \cos x = -\frac{7}{9} and you know x is obtuse, so you really want \cos (\pi - x) = -\cos x = - (-7/9) = 7/9
    Thank you so much! I (eventually) got there! Just to speed up my working in the exam, is there a slightly different way I could approach this to obtain the acute angle originally?
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    (Original post by pippabethan)
    Thank you so much! I (eventually) got there! Just to speed up my working in the exam, is there a slightly different way I could approach this to obtain the acute angle originally?
    Not really. Unless you could see that if you took AB and AC and their dot product was negative. Then you can use BA and AC or AB and CA to get a positive dot product.
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    (Original post by Zacken)
    Not really. Unless you could see that if you took AB and AC and their dot product was negative. Then you can use BA and AC or AB and CA to get a positive dot product.
    Ah right, I might try that in the future
 
 
 
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