Hello, Broadband!
You wrote on Sun, 13 Aug 2006 01:51:01 -0700:

BA I have seen some great posts about colouring the area
BA between 2 lines but I need to calculate numerically the
BA area between 2 lines. A close estimate would do. My curves
BA are smoothed lines but they have many roots to fit a
BA trendline whose equation I could have integrated. If I use
BA straight lines between points I have too big an error.
BA Even Simpsons Rule is not accurate enough at times. Is
BA there a way that Excel can return a figure for the area
BA under a curve - I could even use the number of pixels it
BA has shaded between the 2 lines!

I think from what you say that you realise that the area between
the two curves is the difference between the areas under them.
You could use the differences between the integrals of the
fitted polynomials. With Excel 2002 as I have or earlier, I
would not use the equation given by the trend line on the chart
but actually fit the curve.

I'm not really quite sure what you mean by Simpson's rule not
being accurate enough. It would seem to me that if you have a
large number of experimental points, it is about as accurate as
you can get. In fact, it uses all the information available
unless you have good reason to believe the real graph is smooth.

James Silverton
Potomac, Maryland

E-mail, with obvious alterations:
not.jim.silverton.at.comcast.not