## anonymous 5 years ago I REAAAAAALLLLLLY NEED HELP! :'( express cos80ºcos70º+sin80ºsin70º as a single trigonometric function of a positive acute angle.

1. anonymous

2. anonymous

how did you get that?

3. anonymous

you need to use the double angle formula for cosine: $\cos(\theta-\phi)=\cos(\theta)\cos(\phi)-\sin(\theta)\sin(\phi)$

4. anonymous

You have $\theta = 80, \phi = 70$

5. anonymous

Correction - the sign on the right-hand side should be +, not -.

6. anonymous

7. anonymous

Not really - he asked for an explanation.

8. anonymous

but cool cause my friend needed the formula thanks

9. anonymous

or she...