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    Exercise 6d question 7, chapter 6 c3 edexcel book

    3tan^2theta+4sec^2theta=5

    I have done a different working out and it is wrong . can someone explain why the other working doesn't get me the right answer
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    (Original post by JSM1)
    Exercise 6d question 7, chapter 6 c3 edexcel book

    3tan^2theta+4sec^2theta=5

    I have done a different working out and it is wrong . can someone explain why the other working doesn't get me the right answer
    Well what was the working you did?
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    This is my working out
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    .......
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    i know the other way where you change the sec into tan but what if you dont do it that way. How would you check if you have the right answer in the exam, do you just assume that its the right way?
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    (Original post by JSM1)
    .......
    From \displaystyle \sec^2 \theta = \frac{8}{7}

    to

    \displaystyle \sec \theta = \frac{8}{\sqrt{7}}

    This is wrong. Can you see why?
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    (Original post by JSM1)
    i know the other way where you change the sec into tan but what if you dont do it that way. How would you check if you have the right answer in the exam, do you just assume that its the right way?
    You can change tan->sec or sec->tan and you'll get the right answers either way. Sometimes you can plan ahead to see which option makes life easier.

    You made a mistake later in your working as explained above which is why you got the wrong answers I think.
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    (Original post by Notnek)
    From \displaystyle \sec^2 \theta = \frac{8}{7}

    to

    \displaystyle \sec \theta = \frac{8}{\sqrt{7}}

    This is wrong. Can you see why?
    shouldnt he root the whole thing and not just the denominator?
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    (Original post by JSM1)
    No
    Please quote so it is clear what you are replying to.

    If x^2 = \frac{a}{b} then x=\pm \sqrt{\frac{a}{b}}

    I'm not sure why you only rooted the denominator, plus you missed the \pm.
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    Yes i have done it again and i got the answer. In all cases do you always have to root the whole term on the right because for some questions i have seen that only the denominator gets rooted. Can you explain that please?
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    (Original post by JSM1)
    Yes i have done it again and i got the answer. In all cases do you always have to root the whole term on the right because for some questions i have seen that only the denominator gets rooted. Can you explain that please?
    Do you have any examples that you've seen? It's hard to know what you mean since you always need to root the whole fraction.
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    2sintheta=cosectheta
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    sectheta=2costheta
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    3sec^2theta-4=0
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    (Original post by JSM1)
    2sintheta=cosectheta
    What are you showing here?

    Also please click "reply" to reply to posts so that we get quoted. Thanks.
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    (Original post by Notnek)
    What are you showing here?

    Also please click "reply" to reply to posts so that we get quoted. Thanks.
    These are the examples in which i rooted only the denominator while solving and i got the right answer
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    (Original post by JSM1)
    3sec^2theta-4=0
    For this one you do

    \displaystyle \sec^2 \theta = \frac{4}{3}

    \displaystyle \Rightarrow \sec \theta = \pm\sqrt{\frac{4}{3}} = \pm\frac{\sqrt{4}}{\sqrt{3}} = \pm\frac{2}{\sqrt{3}}

    So you still root the whole fraction but you may be able to simplify like I've done above.
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    (Original post by Notnek)
    For this one you do

    \displaystyle \sec^2 \theta = \frac{4}{3}

    \displaystyle \Rightarrow \sec \theta = \pm\sqrt{\frac{4}{3}} = \pm\frac{\sqrt{4}}{\sqrt{3}} = \pm\frac{2}{\sqrt{3}}

    So you still root the whole fraction but you may be able to simplify like I've done above.
    So in all cases the whole term should be rooted and not just the denominator?
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    (Original post by JSM1)
    So in all cases the whole term should be rooted and not just the denominator?
    Yes.
 
 
 
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