# need help on solving the end of a calculus problem

Discussion in 'Community Discussion' started by macman2790, Mar 25, 2007.

1. ### macman2790 macrumors 6502a

Joined:
Sep 4, 2006
Location:
Texas
#1
I'm using rolle's theorem which doesnt really matter.

All i need to know is how to solve this:

0 = 2*pi*cos(2*pi*x)

I know it deals with an inverse cosine but i dont know how to manipulate it.

thanks.

2. ### plinden macrumors 68040

Joined:
Apr 8, 2004
#2
I'm not going to tell you the answer, but you don't need to worry about the 2*pi multiplier, meaning you're looking for a value of x where cos(2*pi*x) = 0

3. ### P-Worm macrumors 68020

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Jul 16, 2002
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Salt Lake City, UT
#3
Instead of using an inverse cosine (which you still could, it would work), just think to yourself "What value does 2*Pi*x need to be to make the cosine of it equal to zero?"

P-Worm

4. ### GFLPraxis macrumors 604

Joined:
Mar 17, 2004
#4

You've got three elements being multiplied here; 2, pi, and cos(2*pi*x).
For the whole thing to be equal to zero, one of those three elements has to be zero, right? (since anything multiplied by zero is zero).

2 can never be 0. Pi can never be zero. You know that 0 = cos(2*pi*x).

What does X have to be for that to be zero?

5. ### siurpeeman macrumors 603

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Dec 2, 2006
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the OC
#5
or you could ask where cosine is equal to zero on the unit circle.

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Redmond, WA
7. ### siurpeeman macrumors 603

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Dec 2, 2006
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the OC
#7
the x coordinate on the unit circle is a representation of cosine.

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8. ### NightFlight macrumors 6502

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Feb 25, 2007
Location:
Northern Virginia
#8
Oh sweet lord I hated calc.

This makes my head hurt even just trying to remember.

9. ### swiftaw macrumors 603

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Jan 31, 2005
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Omaha, NE, USA
#9
Am I the only one who is confused about why this is referred to as a calculus problem?

Algebra problem, yes. Trigonometry question, yes. Not sure there is any calculus here though.

Edit: Re-reading the OP, I guess it was a calculus problem that yielded the final equation to be solved.

cos(Y) = 0 when Y is an odd multiple of pi/2 that is when Y = (2i - 1)*pi/2 where i is any integer.

thus in your problem cos(2*pi*x) = 0, x = (2i-1)/4 where i is any integer

10. ### BurtonCCC macrumors 65816

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Wheaton/Normal, IL
11. ### iSaint macrumors 603

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May 26, 2004
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South Mississippi y'all, near the water!
#11
I concur.

12. ### Lunja macrumors 6502

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May 15, 2005
Location:
Lincoln UK
#12
Nope, the answer's 42. It's the question that's wrong

13. ### mkrishnan Moderator emeritus

Joined:
Jan 9, 2004
Location:
Grand Rapids, MI, USA
#13
Clearly you are confused, but not as confused as the person with the dog.

I'm guessing the OP had to solve this as part of a calculus problem. Of course this aspect is pretty purely trig.

14. ### Abstract macrumors Penryn

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Dec 27, 2002
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Location Location Location
#14
How can you be doing calculus when you can't solve a very fundamental trigonometry problem?

15. ### mkrishnan Moderator emeritus

Joined:
Jan 9, 2004
Location:
Grand Rapids, MI, USA
#15
No one said anything about doing calculus well.