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    Where have i gone wrong?
    Show that L.H.S can be written as R.H.S ..........x is meant to be theta

    (cosx-sinx)(17cosx-7sinx) = 5cos2x-12sin2x+c

    17cos^2x-7sinxcosx-17cosxsinx+7sin^2x

    17cos^2x+7sin^2x-24sinxcosx

    10cos^2x-24sinxcosx+7

    5(2cos^2x-1)-12sin2x+7

    10cos^2x-5-12sin2x+7 - think this is wrong

    please help
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    (Original post by joyoustele)
    Where have i gone wrong?
    Show that L.H.S can be written as R.H.S ..........x is meant to be theta

    (cosx-sinx)(17cosx-7sinx) = 5cos2x-12sin2x+c

    17cos^2x-7sinxcosx-17cosxsinx+7sin^2x

    17cos^2x+7sin^2x-24sinxcosx

    10cos^2x-24sinxcosx+7

    5(2cos^2x-1)-12sin2x+7

    10cos^2x-5-12sin2x+7 - think this is wrong

    please help
    Check that the 4th line of working expands to give the 3rd line - you've made a small mistake.

    Then use \cos 2x \equiv 2\cos^2 x - 1, which is one of the forms of the \cos 2x identity that you'll need to know. I'm not sure why you've expanded again in your final line.
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    (Original post by Notnek)
    Check that the 4th line of working expands to give the 3rd line - you've made a small mistake.

    Then use \cos 2x \equiv 2\cos^2 x - 1, which is one of the forms of the \cos 2x identity that you'll need to know. I'm not sure why you've expanded again in your final line.
    10cos^2x-24sinxcosx+7

    5(2cos^2x)-24sinxcosx+7

    Then?..........
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    (Original post by joyoustele)
    10cos^2x-24sinxcosx+7

    5(2cos^2x)-24sinxcosx+7

    Then?..........
    Well you could just rearrange this identity

    \cos 2x = 2\cos^2 x - 1

    to make \cos^2 x the subject and then just sub that in. That's one way of doing it.
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    (Original post by Notnek)
    Well you could just rearrange this identity

    \cos 2x = 2\cos^2 x - 1

    to make \cos^2 x the subject and then just sub that in. That's one way of doing it.
    Wow, never thought of that
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    Please check the attachment


    (Original post by joyoustele)
    Where have i gone wrong?
    Show that L.H.S can be written as R.H.S ..........x is meant to be theta

    (cosx-sinx)(17cosx-7sinx) = 5cos2x-12sin2x+c

    17cos^2x-7sinxcosx-17cosxsinx+7sin^2x

    17cos^2x+7sin^2x-24sinxcosx

    10cos^2x-24sinxcosx+7

    5(2cos^2x-1)-12sin2x+7

    10cos^2x-5-12sin2x+7 - think this is wrong

    please help
    Attached Images
     
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    (Original post by gurungg48)
    Please check the attachment
    Nice, what tool did you use when answering the question?
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    (Original post by joyoustele)
    Nice, what tool did you use when answering the question?
    I Use Idroo. Its really good. I use this to teach online too.
 
 
 
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