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January 25, 2015

January 25, 2015

Posted by **Paul** on Monday, December 1, 2008 at 8:19pm.

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

- trig -
**Reiny**, Monday, December 1, 2008 at 8:25pmhave 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 -
**Paul**, Monday, December 1, 2008 at 8:28pmNo I have not come across that before.

- trig -
**Reiny**, Monday, December 1, 2008 at 8:44pmthen 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 -
**Paul**, Monday, December 1, 2008 at 8:48pmOh I get it now. thanks for your help.

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