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Exponential and logarithmic graphs/functions HELP

Hi.

I have a maths assignment due in 3 days and i have run into trouble half way through. The basic run down of the assignment is that we have to create an exponential function (with a limit (+c)) from real life data (world record times for triple jump going back 50 years). We are required to convert the data using natural log (ln) to produce a linear graph in which taking that gradient, using it to form an exponential graph.

The data if a regression line is run through them (in excel), forms a line following y=(-a)e^(-bx) whilst the equation found using the ln route gives an equation following y=(a)e^(bx). At this point, there still is no limit thus no asymptote (which is required).

I am having trouble revising the equation still using the ln step (because its required) to form to y=(-a)e^(-bx).

Attached is a .doc of my assignment (the .xls is my working sheet)

Can someone tell me what im doing wrong? The assignment is due on Monday (18/8/08) and i would really appreciated if someone could help me before then.

Thanks.