exp fit problem

[mimatopo]

Hi together,
i do need to calculate an exponential fit to my data to finally get out the
lamda. In matlab, this is no problem since I can use the 'fit'-option. I
tried some possibilities (e.g. expfit), but I failed :-(
maybe some of you can help me:
I have two columns of data, e.g:
 
x       y
18      20
16      30
13      40
11      60
9       80
7       100
5       150
3       220
How can I calculate a fit according a general exp model according to
f(x) = a*exp(b*x)
... I need the calculated b for this curve...
 
I hope sombody can help me with this!
 
Thnaks a lot and best regards,
Tom
 




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