# Another trig question

• Feb 3rd 2010, 11:41 PM
ceroseven
Another trig question
Value of cos65°cos12°+sin65° sin12°

-The Answer must be in either sin or cos and must be followed with a degree (i.e. cos88°)

1.6
• Feb 4th 2010, 12:00 AM
VonNemo19
Quote:

Originally Posted by ceroseven
Value of cos65°cos12°+sin65° sin12°

-The Answer must be in either sin or cos and must be followed with a degree (i.e. cos88°)

1.6

You want the seccend sum or difference identity, here

Table of Trigonometric Identities
• Feb 4th 2010, 12:15 AM
ceroseven
How would this be substituted in the equation?
• Feb 4th 2010, 09:31 PM
VonNemo19
Quote:

Originally Posted by ceroseven
How would this be substituted in the equation?

Let $u=65^\circ$ and let $v=12^\circ$, then your problem reduces to

$\cos65^\circ\cos12^\circ+\sin65^\circ\sin12^\circ= \cos(65^\circ-12^\circ)=\cos53^\circ$