How do I evaluate this properly? Trig

[aisha]

I have the question evaluate the following : cos(-pi/12)

I rewrote this to be -15 degrees = 345 degrees.

I think I am supposed to use the sum and difference formula or special triangle, so I wrote

cos(30degrees-45degrees)= cos 30 cos 45 - sin 30 sin 45

my answer turned out to be 0.5124 but it says the correct answer is 0.9659 what did I do wrong?

 

[dextercioby]

I have the question evaluate the following : cos(-pi/12)

I rewrote this to be -15 degrees = 345 degrees.

I think I am supposed to use the sum and difference formula or special triangle, so I wrote

cos(30degrees-45degrees)= cos 30 cos 45 - sin 30 sin 45

my answer turned out to be 0.5124 but it says the correct answer is 0.9659 what did I do wrong?

U applied the 'cosine' formula incorrectly.It should read
cos(30degrees-45degrees)= cos 30 cos 45 +sin 30 sin 45

I think it gives you the right answer.

 

[aisha]

Ok I changed the sign but now my answer is 1.219 it is still wrong

 

[Yapper]

You added wrong, aisha

 

[aisha]

yes ur right thanks i didnt do it separately I just input the whole thing into the calculator thanks