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# Exact Values Question watch

1. 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)

2. (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)

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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Updated: October 29, 2017
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