# NCEES Power #115



## mike123 (Mar 20, 2009)

Anyone know how to enter the below formula work out the area under the curve using the CASIO fx-115MS

Answer = 1/pi * integrate Sin(x) between 45deg and 180deg ?

Answer = 0.543A


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## Flyer_PE (Mar 20, 2009)

I think you are actually asking about question #115.

Not knowing anything about the Casio, I have to ask why you wouldn't just do the integration manually?

The integral of sin x is -cos x

All you need are the value for pi, cos 45o, and cos 180o

cos 180o = -1

cos 45o = 1/sqrt2

It's simple algebra from there.


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## mike123 (Mar 21, 2009)

Good spot. The question number indeed should read #115 and not 114. I would fix but I can't figure out how to do it.

Just though I should learn how to use the calculator as a way to double check the answer, and to save a precious minute. Especially if there are a number of integration problem during the exam. The minute saved adds up.


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## geofs_PE (Mar 21, 2009)

First, change to radians: [shift] [Mode] [4]

Then:



```
∫(sin(X),(π/4), π)

ANS/ π =
```

The '/' is the 'divided by' operator, I don't seem to have a symbol for it.

The time it takes the calculator to solve the integral is probably ~60 sec. or so, so be patient.

This gives the correct answer but I agree with Flyer, it's faster and easier to do the integration manually.


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## BamaBino (Sep 27, 2010)

If this problem (115) had instead asked for the RMS value of the current, would the correct answer be 0.674 amps?


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## Flyer_PE (Sep 27, 2010)

mike123 said:


> Good spot. The question number indeed should read #115 and not 114. I would fix but I can't figure out how to do it.


Title updated.


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## ChrisB (Nov 13, 2020)

Coming back to this question, why do they use radians (1/pi) prior to the integral, but then degrees (45 - 180) after the integral sign? The handbook shows everything being in radians (before and after the integral) for the equation. When using radians for the entire equation, I'm getting a different answer than 0.543A. I'm getting 0.00044A.


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