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March 3, 2015

March 3, 2015

Posted by **lan** on Sunday, May 8, 2011 at 9:56am.

- trig -
**Reiny**, Sunday, May 8, 2011 at 10:23amstart with

cos 2A = 2cos^2 A - 1

cos150° = 2 cos^2 75 - 1

-√3/2 + 1 = 2cos^2 75°

cos^2 75° = (1 - √3/2)/2

cos 75° = √[(1 - √3/2)/2]

An easier ways would have been

cos75 = cos(45+30)

= cos45cos30 - sin45sin30

= (1/√2)(√3/2) - (1/√2)(1/2)

= (√3 - 1)/(2√2)

Use your calculator to show that both produce cos75

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