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 Solve this equation for x
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 Return to top of page · Post #: 1 · Written at 3:57:31 AM on 10 January 2015.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6005

Folks,

Please solve this equation for x and let me know your answer:

x = 240.7263/(7.591386/log10(67.04/6.116441)-1)

Note: The -1 at the end is an exponent.

I get a result that does not agree with the textbook, but I may be making an error.


 
 Return to top of page · Post #: 2 · Written at 8:26:40 AM on 10 January 2015.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4337

Did you work out what was in the brackets first? Strange way of putting the equation? Almost looks like an escapee from a "slide rule" (got two)

log base 10 is the common logarithms.

Marc


 
 Return to top of page · Post #: 3 · Written at 9:26:17 AM on 10 January 2015.
Tinkera123's Gravatar
 Location: Melbourne, VIC
 Member since 5 October 2009
 Member #: 555
 Postcount: 410

-32.97

Ian


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
Cheers, Ian

 
 Return to top of page · Post #: 4 · Written at 2:26:07 PM on 10 January 2015.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
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No result will come from me unfortunately. Equations were probably my weakest point in maths at school and maths was generally a subject I loathed apart from geometry.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 5 · Written at 3:32:19 PM on 10 January 2015.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6005

.Brad: Maths was never my strong subject. I had to work hard to concentrate as my mind always said: "You'd rather be doing anything else than this".

.Marcc: That's how the equation is given in the text, and I'm using the common log function. When I put it in Excel in the format as shown I got a different result from the text, so I executed it in single steps from the inside out, following the parentheses, and still differ.

.Ian: Thanks. Via Excel I get 30.49565355. The book says: 38.21


Anybody able to get 38.21?


 
 Return to top of page · Post #: 6 · Written at 4:15:52 PM on 10 January 2015.
Tinkera123's Gravatar
 Location: Melbourne, VIC
 Member since 5 October 2009
 Member #: 555
 Postcount: 410

Hi GTC,

I will check my calculation, but it shouldn't be a difficult equation to solve. Also did it "inside out" with Excel. Could be interpretation of formula from the text. Can you email the actual text?

Cheers,
Ian


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
Cheers, Ian

 
 Return to top of page · Post #: 7 · Written at 4:24:33 PM on 10 January 2015.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6005

Okay, image of text emailed. (two graphics).


 
 Return to top of page · Post #: 8 · Written at 5:40:02 PM on 10 January 2015.
Tinkera123's Gravatar
 Location: Melbourne, VIC
 Member since 5 October 2009
 Member #: 555
 Postcount: 410

GTC,

The equation is open to various interpretations.

I intrepreted the exponent as log{(X)-1} .... and being less than 1 ... yields a minus answer.

If the exponent is of the whole log function ie {log(X)}-1 then I also get 30.4956.....

Treating the -1 as integer "minus 1" ..... doesn't help either.

Another possibility, besides the book being incorrect (and it wouldn't be the first time), is the interpretation of "10log". I would also assume 'log base 10', but other bases to logs are used.
Is the text specific about "log base 10"??

Cheers,
Ian


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
Cheers, Ian

 
 Return to top of page · Post #: 9 · Written at 6:00:48 PM on 10 January 2015.
Tinkera123's Gravatar
 Location: Melbourne, VIC
 Member since 5 October 2009
 Member #: 555
 Postcount: 410

Hey GTC,

Just got your second email ..... the book is correct .... 32.21.

The problem was with the interpretation of -1.

Calculation should be Tn/[{m/log(Pw/A)}-1] ie subtract 1 from m/log(Pw/A) then divide into Tn

Log is base 10

I will email my spreadsheet ... see the last calculation.

Cheers,
Ian


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
Cheers, Ian

 
 Return to top of page · Post #: 10 · Written at 6:07:49 PM on 10 January 2015.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6005

Much confusion for me! Many thanks for the worked example.

I see you mean to say 38.21 above.


 
 Return to top of page · Post #: 11 · Written at 6:51:10 PM on 10 January 2015.
Tinkera123's Gravatar
 Location: Melbourne, VIC
 Member since 5 October 2009
 Member #: 555
 Postcount: 410

Yes, 38.21. May I ask what the temperature calculation is for? Just curious.

Ian


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
Cheers, Ian

 
 Return to top of page · Post #: 12 · Written at 9:22:24 PM on 10 January 2015.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6005

It's supposed to allow calculation (within 1% accuracy) of dew point temperature without recourse to the psychometric chart. I now have to see how good it is with local data.


 
 Return to top of page · Post #: 13 · Written at 9:43:22 PM on 10 January 2015.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
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 Postcount: 4337

The only way you get 38.21 is to treat -1 as minus 1

If you divide out that which is in the inner bracket its log is 1.0398. divide 7.59.... by that and subtract 1 you end up with 6.3 as the divisor so then you have 240.72... / 6.3

Marc


 
 Return to top of page · Post #: 14 · Written at 10:18:29 PM on 10 January 2015.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6005

Marcc, yes thanks, Ian set me straight on that in post #9 above.

All good now.


 
 Return to top of page · Post #: 15 · Written at 9:00:29 AM on 11 January 2015.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4337

Basically, if the formula was written properly, there would never have been an issue: Only a problem, the value of "X"


 
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