# Sum and differences identities equations

## Homework Statement

Which is equivalent to: cos(∏/2 + x) - cos(∏/2 - x)?
A) -2cos(x)
B) -2
C) 0
D)-2sin(x)

Cos (A-B)

Dick
Homework Helper

## Homework Statement

Which is equivalent to: cos(∏/2 + x) - cos(∏/2 - x)?
A) -2cos(x)
B) -2
C) 0
D)-2sin(x)

Cos (A-B)

## The Attempt at a Solution

You mentioned the sum and difference formulas. I think you should try to use them.

You mentioned the sum and difference formulas. I think you should try to use them.

I have tried to use them but i totally get stuck like i dont know which is a or b and we are only given cos (a-b) = cosAcosB + sinAsinB

Dick
Homework Helper
I have tried to use them but i totally get stuck like i dont know which is a or b and we are only given cos (a-b) = cosAcosB + sinAsinB

In cos(pi/2-x) the 'a' is pi/2 and the 'b' is x. So what does that turn into? What are cos(pi/2) and sin(pi/2)?

Cos (pi/2) =0
Sin(pi/2)= 1

Dick
Homework Helper
Cos (pi/2) =0
Sin(pi/2)= 1

Ok, do go on. So what is cos(pi/2-x)?

How come the b is not a -x ?

So cos( 0-x)?

SteamKing
Staff Emeritus
Homework Helper
I have tried to use them but i totally get stuck like i dont know which is a or b and we are only given cos (a-b) = cosAcosB + sinAsinB

It doesn't matter. Pick one angle and make it A. The other then must be B.

Dick
Homework Helper
How come the b is not a -x ?

Why would you think that?? If you have a formula for cos(a-b) and you want to apply it to cos(pi/2-x) then you should put a=pi/2 and b=x. Your formula already has the '-' in it. Just use the formula and stop trying to double think it.

Why would you think that?? If you have a formula for cos(a-b) and you want to apply it to cos(pi/2-x) then you should put a=pi/2 and b=x. Your formula already has the '-' in it. Just use the formula and stop trying to double think it.

But the formula also has pi/2 + x theres 2 A's and 2 B's

Dick