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    Hi,

    It's been a few years since I've done my higher maths so I'm having some trouble remembering how to do things. Can someone explain to me how I would do something like this:

    "7. If the exact value of cos is 1/5 find the exact value of cos2x.

    A. (-3)/5

    B. (-2)/5

    C. 2/5

    D. 3/5

    (Sorry for the bad fractions but I don't know how else to put them into the question)

    Thx in advance
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    (Original post by CallumMort)
    Hi,

    It's been a few years since I've done my higher maths so I'm having some trouble remembering how to do things. Can someone explain to me how I would do something like this:

    "7. If the exact value of cos is 1/5 find the exact value of cos2x.

    A. (-3)/5

    B. (-2)/5

    C. 2/5

    D. 3/5

    (Sorry for the bad fractions but I don't know how else to put them into the question)

    Thx in advance
    Are u familiar with double angle formulae.

    Cos(2x)=2((cos(x))^2)-1.

    My formatting is also bad, hope u can decipher that.

    Let me know if ur familiar with this.
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