There are two function in excel that you need. Slope and intercept. I will
assume they need no expanation. Assume your co-ordinates look like this
(starting in Cell A1):

Point X Point Y
1 3
2.5 4
3 5

=B4-(A4*SLOPE(B2:B3, A2:A3)+INTERCEPT(B2:B3, A2:A3))

This shows that the two sets are not co-linear by .667. Remove the B4 and
you will see where the intercept should be...

HTH

" wrote:

I have sets of coordinates which contain
3 points in each set.

Can anyone suggest how I can test if
all 3 points are collinear (to within say 3 or 4 decimal places)

Or can anyone show me how to simply display
the formula for the line using each pair of points in turn
and displaying all 3 formulas. In the form y=mx+b

Any help would be much appreciated.

Thanks

CharlesW