# Error in book or my error?

Discussion in 'Homework Help' started by Petrovic, Aug 10, 2012.

1. ### Petrovic Thread Starter New Member

Aug 10, 2012
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Hi, I just registered. I am a bit stucked at an example exercise. When I work out the integral (bottom of the picture) I get -V/jnπ. The two zero-limits of the integrals give both -V/jnπ , the other limits should give -2sin(nπ)/jnπ which always is zero.

Because the resolution of the above picture is a bit low I link to an extra picture.

I always assume at first that I make a mistake when I get a different result but I can't find any mistake.

@Moderators: I guess it would be better if the title would be: electronic circuits - error in book or my error?

Last edited: Aug 10, 2012
2. ### WBahn Moderator

Mar 31, 2012
17,461
4,701
The book answer given is correct. We can't begin to point out where you made your mistake unless you show us your work.

3. ### Petrovic Thread Starter New Member

Aug 10, 2012
4
0
My attempt.

An earlier example in the book:

Last edited: Aug 11, 2012
4. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Your working is correct thus far. Try evaluating your result at any odd value of n and your answer will agree with the book result.

5. ### WBahn Moderator

Mar 31, 2012
17,461
4,701
You're doing fine. You just need to finish things.

You could massage things to replace the complex exponentials with a trig formula, or you could just take note of the fact that 'n' has to be an integer and consider what the results are when n is even and when n is odd.

You're very close.

6. ### Petrovic Thread Starter New Member

Aug 10, 2012
4
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I think I understand it.
When n is odd you get -1 for e^j*n*pi = cos(n*pi) + j*sin(n*pi) because cos(n*pi) = -1 and sin(n*pi) = 0 for n is odd. The same for e^-j*n*pi = cos(-n*pi) + j*sin(-n*pi).

7. ### WBahn Moderator

Mar 31, 2012
17,461
4,701
Yep, that'll do for when n is odd.

Be sure you do the same thing for n even to see that those terms vanish.

8. ### Petrovic Thread Starter New Member

Aug 10, 2012
4
0
I did. Thanks.