barycentre takes as argument two lists of the same size (resp. a matrix with two columns):
barycentre returns the barycenter point of the points Aj
weighted by the real coefficients αj.
If ∑αj = 0, barycentre returns an
Warning To have a complex number in the output, the input must be :
affix(barycentre(...,...)) because barycentre(...,...) returns a point, not a complex number.