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March 2, 2015

March 2, 2015

Posted by **Abby** on Sunday, February 28, 2010 at 11:39am.

cos^2 18 degrees + cos^2 72 degrees.

- Math(Please check) -
**Abby**, Sunday, February 28, 2010 at 11:41amError

- Math(Please check) -
**Reiny**, Sunday, February 28, 2010 at 12:05pmThe cofunction identities that are useful here are

cos(90-x) = sinx and

sin(90-x) = cosx

and the fact that cos(A-B) = cosAcosB + sinAsinB

so cos^2 72 = (cos72)(cos72) = (sin18)(sin18)

then your

cos^2 18 degrees + cos^2 72 degrees

= cos18cos18 + sin18sin18

= cos(18-18)

= cos 0

= 1

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