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OCR Core 2 Jan 2008 part ii Watch

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    I am writing up some solutions.
    This is a Cos rule Question. We agree on part (i)
    Name:  Screen Shot 2016-02-16 at 16.32.17.png
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    For part ii I get 0.4036 as the Cos of the Angle. The mark scheme gets 0.3998.

    Here is my working 

\cos \text{BAD}=\frac{20^2+10^2-18.4^2}{2\times20\times 10}=0.4036

    Here is their's
    Section A 																18.4^2 =10^2 +20^2 –2 \times10 \times 20 \times \cos \theta					>>>> cos \theta = 0.3998

    Where am I going wrong? Marks schemes are usually right.
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    You need to use the exact length of BD from your calculator, not the rounded value of 18.4. This then gives 0.3998 as expected.
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    (Original post by nerak99)
    I am writing up some solutions.
    This is a Cos rule Question. We agree on part (i)
    Name:  Screen Shot 2016-02-16 at 16.32.17.png
Views: 33
Size:  44.0 KB

    For part ii I get 0.4036 as the Cos of the Angle. The mark scheme gets 0.3998.

    Here is my working 

\cos \text{BAD}=\frac{20^2+10^2-18.4^2}{2\times20\times 10}=0.4036

    Here is their's
    Section A 																18.4^2 =10^2 +20^2 –2 \times10 \times 20 \times \cos \theta					>>>> cos \theta = 0.3998

    Where am I going wrong? Marks schemes are usually right.
    As written, the mark scheme appears to be wrong, due to rounding somewhere I guess.
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    No, it's right if you use the exact value, which is 18.44169958106291476690091790895 6. This then gives cos theta =
    0.399759291404559784945513251979 7
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    (Original post by constellarknight)
    No, it's right if you use the exact value, which is 18.44169958106291476690091790895 6. This then gives cos theta =
    0.399759291404559784945513251979 7
    Maybe, but that doesn't seem to be in the mark scheme if the OP has written it out faithfully.
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    Yes, the mark scheme is a bit confusing in that it "pretends" to use the rounded value of just 18.4, which is incorrect. Anyway hopefully nerak99 now understands the correct solution.
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    Well sometimes you just can't see for looking. Thanks
 
 
 
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