Homework Help: Trigonometry

Posted by Kayla on Tuesday, March 22, 2011 at 10:03pm.

Write the expression as a trigonometric function of one number.
(cos 20 degrees)(cos 70 degrees)+(sin 20 degrees)(sin 70 degrees)

Trigonometry - Reiny, Tuesday, March 22, 2011 at 10:06pm
Have you come across the identity

cos(A-B) = cosAcoB + sinAsinB ?

your question fits that pattern perfectly.

Let me know what you get.
Trigonometry - Kayla, Tuesday, March 22, 2011 at 10:11pm
ok I made a mistake in the formula. it's cos(20 degrees)cos(70 degrees)- sin(20 degrees) sin(70 degrees).

so it's a subraction equation not an addition.
Trigonometry - Reiny, Tuesday, March 22, 2011 at 10:13pm
and of course there is the matching formula for

cos(A+B) = cosAcosB - sinAsinB

So what do you think?
Trigonometry - Kayla, Tuesday, March 22, 2011 at 10:15pm
so it would be cos(20+70)?
Trigonometry - Kayla, Tuesday, March 22, 2011 at 10:16pm
cos(20 degrees+70 degrees) i mean
Trigonometry - Reiny, Tuesday, March 22, 2011 at 10:38pm
yes, but what is cos(20° + 70°) ??
= cos 90°
= 0

so your
cos(20 degrees)cos(70 degrees)- sin(20 degrees) sin(70 degrees)
= 0
Trigonometry - Kayla, Tuesday, March 22, 2011 at 10:49pm
Yeah I got the exact value I just needed it in trigonometric function of one number which would be the degrees of cos.

Thanks!!!
Trigonometry - girl25, Friday, April 15, 2011 at 10:12pm
HELP ME !

sin50 cos 20 - cos 50 sin 20
^find exact value
Trigonometry - Sunny, Tuesday, July 26, 2011 at 1:32am
Pls tell me how to find out sin70.Pls

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