Sum and differences identities equations

  • Thread starter keishaap
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  • #1
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Homework Statement


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


Homework Equations


Cos (A-B)


The Attempt at a Solution


I am totally stuck :( please help!
 

Answers and Replies

  • #2
Dick
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Homework Statement


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


Homework Equations


Cos (A-B)


The Attempt at a Solution


I am totally stuck :( please help!

You mentioned the sum and difference formulas. I think you should try to use them.
 
  • #3
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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
 
  • #4
Dick
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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)?
 
  • #5
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Cos (pi/2) =0
Sin(pi/2)= 1
 
  • #6
Dick
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Cos (pi/2) =0
Sin(pi/2)= 1

Ok, do go on. So what is cos(pi/2-x)?
 
  • #7
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How come the b is not a -x ?
 
  • #9
SteamKing
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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.
 
  • #10
Dick
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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.
 
  • #11
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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
 
  • #12
Dick
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But the formula also has pi/2 + x theres 2 A's and 2 B's

Use the formula SEPARATELY for each one. You can choose them differently for cos(pi/2-x) and cos(pi/2+x). Finish cos(pi/2-x) first, then worry about the other one.
 
  • #13
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Use the formula SEPARATELY for each one. You can choose them differently for cos(pi/2-x) and cos(pi/2+x). Finish cos(pi/2-x) first, then worry about the other one.

Okay i thought they had to be the same because the teacher didn't show us any other way thanks!
 

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