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problem with math.log function in vb.net

Posted on 2008-10-19
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Last Modified: 2010-04-21
I am not sure if I am using the math.log function in vb.net properly.

I am trying to do the formula
%Fat=495/(1.0324-0.19077(log(waist-neck))+0.15456(log(height)))-450
(and converting inches to cm)
with
  LbJP.Text = 495 / (1.0324 - 0.19077 * (Math.Log((Val(TxtWaistBL.Text) * 2.54) - (Val(TxtNeck.Text) * 2.54))) + 0.15456 * (Math.Log((Val(TxtHeight.Text) * 2.54)))) - 450
but the number isn't coming out anywhere near close to where it should.

High points for expediency! I need an answer quickly
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Question by:JCM0165
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Expert Comment

by:GreymanMSC
ID: 22754151
The Body Fat Calculation uses base 10 logarithm, where as Math.Log gives the Natural Logarithm.  They are not the same thing.

To convert to base 10 log, you need to divide by the natural log of 10.
  LbJP.Text = 495 / (1.0324 - 0.19077 * (Math.Log((Val(TxtWaistBL.Text) * 2.54)/Math.Log(10) - (Val(TxtNeck.Text) * 2.54))) + 0.15456 * (Math.Log((Val(TxtHeight.Text)/Math.Log(10) * 2.54)))) - 450

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Accepted Solution

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GreymanMSC earned 500 total points
ID: 22754162
Duh...
LbJP.Text = 495 / (1.0324 - 0.19077 * (Math.Log((Val(TxtWaistBL.Text) * 2.54) - (Val(TxtNeck.Text) * 2.54))/Math.Log(10)) + 0.15456 * (Math.Log((Val(TxtHeight.Text) * 2.54))/Math.Log(10))) - 450

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Expert Comment

by:GreymanMSC
ID: 22754168
Make sure the bracketing around the logs is in the right place.

The important thing is that:

  Log10(X) = Math.Log(X) / Math.Log(10)
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Author Closing Comment

by:JCM0165
ID: 31507663
Thanks!
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