Question

verify cos(a+b)cos(a-b) = cos^2b-sin^a

Answer 1

do you want to PROVE the sum formula?

Answer 2

http://openstudy.com/users/enlightened#/updates/534ad228e4b0dfc5ac71df10
looks like you asked that question here
and freckles gave you great advice

Answer 3

and difference formula*

Answer 4

I think the distributive property answer is really simple

Answer 5

i am going to ask you the same question freckles asked
can you give me what cos(a+b) is equal to and what cos(a-b) is equal to?

Answer 6

Hint: cos(a+b) = cos(a+b) because of the property x =x

Answer 7

LOL I think he means in terms of cos(a) and cos(b) and sin(a) and sin(b)

Answer 8

@Enlightened if you want my help can you please answer my question?

Answer 9

oh i'm sorry, yeah it's in terms of cos a and cos b and sina and sinb, there are no actual values to them.

Answer 10

...those are the terms, you must relate them.

Answer 11

@enlightened will you answer my question.
I'm trying to get you to use the sum and difference identities on cos(a+b) and cos(a-b)
you have not done that yet.

Answer 12

cos(a+b)=cos(a)cos(b)-sin(a)sin(b)
cos(a-b)=cos(a)cos(b)+sin(a)sin(b)

Answer 13

@myininaya there it is ^

Answer 14

great now multiply those

Answer 15

what do i multiply?

Answer 16

cos(a+b)cos(a-b)
multiply the expansions for these that you gave me

Answer 17

this is what freckles was talking about (c-d)(c+d)=c^2-d^2

Answer 18

I don't understand, where does c-d,c+d come from and do use distributive to multiply all those terms?

Answer 19

multiply cos(a+b)cos(a-b) by using the expansions you gave me

Answer 20

it would be helpful to memorize (c-d)(c+d)=c^2-d^2 though

Answer 21

you gave me this
cos(a+b)=cos(a)cos(b)-sin(a)sin(b)
cos(a-b)=cos(a)cos(b)+sin(a)sin(b)

all I'm asking you to do right now is multiply
(cos(a)cos(b)-sin(a)sin(b))(cos(a)cos(b)+sin(a)sin(b))

Answer 22

this is in the form (c-d)(c+d) which equals c^2-d^2

Answer 23

where c= and d=?