Finding the mean

This is how it is set for task 3 b) I got right just a) I got wrong.
 

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After looking through textbook provided. I thought I would try with 'u substitution' and got a different answer again.
 

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You forgot that u is in radians, not degrees! You can't subtract 360's from it; if anything, it would be 2pi's.

But if you just evaluate the cosine of 1583.362697..., you'll find that it is 1. (Even with your rounded value, you'll get 0.9999999996.)

Using a different (valid) method to evaluate an integral can't change the answer! It is still 0.

If someone or something is telling you that 0 is the wrong answer, then you'll have to ask why. The best bet is that they mean something other than the literal mean (perhaps the mean absolute voltage). But taking the question at face value, the answer is 0.
 
Dr.Peterson said:
You forgot that u is in radians, not degrees! You can't subtract 360's from it; if anything, it would be 2pi's.

But if you just evaluate the cosine of 1583.362697..., you'll find that it is 1. (Even with your rounded value, you'll get 0.9999999996.)

Using a different (valid) method to evaluate an integral can't change the answer! It is still 0.

If someone or something is telling you that 0 is the wrong answer, then you'll have to ask why. The best bet is that they mean something other than the literal mean (perhaps the mean absolute voltage). But taking the question at face value, the answer is 0.
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Thank you for your help, no one is telling me that the method used to get to 0 is wrong. I just want to be sure before submitting my answer. I'm using an online course and I'm not getting any feedback to the questions I'm asking. Thanks again for your help I think I'll go with the 0.
 
Jakeboulter said:
Thank you for your help, no one is telling me that the method used to get to 0 is wrong. I just want to be sure before submitting my answer. I'm using an online course and I'm not getting any feedback to the questions I'm asking. Thanks again for your help I think I'll go with the 0.
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I think in this problem, the "implied" unit of 't' is seconds. Thus, the limits of integration should be 0 to 0.0036. In that case the "mean" will not be 0.
 
Last ditch attempt I'm hoping this is the one.
 

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What do you mean by "total"? My concern would be whether the mean is greater than the maximum, 220. (It isn't.)

Another check would be to compare 0.0036 with the period, to get a sense of how large to expect the mean to be, graphically. What you'll find is that it's about a quarter cycle, which might give you an idea of the result to expect.

(I apologize for not having noticed the unit issue. In your context, seconds is probably an assumed default, but in math it would be an error on their part not to define the variable as measured in seconds, which would have clarified the problem.)
 
Dr.Peterson said:
What do you mean by "total"? My concern would be whether the mean is greater than the maximum, 220. (It isn't.)

Another check would be to compare 0.0036 with the period, to get a sense of how large to expect the mean to be, graphically. What you'll find is that it's about a quarter cycle, which might give you an idea of the result to expect.

(I apologize for not having noticed the unit issue. In your context, seconds is probably an assumed default, but in math it would be an error on their part not to define the variable as measured in seconds, which would have clarified the problem.)
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Sorry by total I ment total voltage but was looking at the wrong bit I've attempted this a different way so many times its thrown me. It's the last question in the unit I need to get right. My new mean now is 140.6899...
(No need to apologise this forum has helped massively with my understanding on how maths questions are implied when written out.
 
That's what I get, and it makes sense because it is a significant fraction of 220.

You may possibly have seen that the mean absolute value of a sinusoid is 2/pi = 0.64 times the peak; and that's just what 140 is in this case, because we're averaging over a quarter cycle.
 
Dr.Peterson said:
That's what I get, and it makes sense because it is a significant fraction of 220.

You may possibly have seen that the mean absolute value of a sinusoid is 2/pi = 0.64 times the peak; and that's just what 140 is in this case, because we're averaging over a quarter cycle.
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So that ended up being wrong aswell. I've not been given the answer so not sure what it could be. I'll just have to live with a Merit rather than distinction for that unit.
 
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