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

- Trigonometry -
**Staciefu**, Tuesday, June 10, 2014 at 7:45pm
How are the sun 70 degrees and cos 20 seg related

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