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trig

posted by .

How do I solve this?

If cos x= 3/5, find the exact value of cos 2x

  • trig -

    have you come across the identity
    cos 2x = 2cos^2 x - 1

    we can just apply this here.
    cos 2x = 2(3/5)^2 - 1
    = -7/25

  • trig -

    No I have not come across that before.

  • trig -

    then probably
    cos2x = cos^2 x - sin^2 x

    (I replaced sin^2 x with 1-cos^2 x to get the one I used)

    Your question fits right into the application of the double and half angle formulas, and without these your question becomes a mess.

  • trig -

    Oh I get it now. thanks for your help.

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