@acer 

Thank you so much for your help. I'm okay now.

@acer 

Thank you

@acer 

Thank you. I have sorted this out already.

@mmcdara 

Please, just ignore my last comment. I understand it now.

@mmcdara 

Thank you for this. I wanted something like after I must have collected the coefficient for both 

(alpha[1]*cos(theta(x, y, t))^2*sin(theta(x, y, t))^2+(alpha[2]+alpha[3]+alpha[6]-alpha[5])*((1/4)*cos(theta(x, y, t))^2-(1/4)*sin(theta(x, y, t))^2)+(1/2)*alpha[4]+(alpha[5]+alpha[6])*((1/4)*cos(theta(x, y, t))^2+(1/4)*sin(theta(x, y, t))^2)+(alpha[3]-alpha[2])*((1/4)*cos(theta(x, y, t))^2+(1/4)*sin(theta(x, y, t))^2))*dudy^2   + (alpha[1]*cos(theta(x, y, t))^2*sin(theta(x, y, t))^2+(alpha[2]+alpha[3]+alpha[6]-alpha[5])*(-(1/4)*cos(theta(x, y, t))^2+(1/4)*sin(theta(x, y, t))^2)+(1/2)*alpha[4]+(alpha[5]+alpha[6])*((1/4)*cos(theta(x, y, t))^2+(1/4)*sin(theta(x, y, t))^2)+(alpha[3]-alpha[2])*((1/4)*cos(theta(x, y, t))^2+(1/4)*sin(theta(x, y, t))^2))**dvdx^2 

But it doesn't collect the coefficients of *dvdx^2.

@mmcdara 

Thank you for this. I don't want them separate. I want to collect the coefficient of dudy^2 and dvdx^2 in the bigger equation. I have been able to collect them separately but this not what I want. 

Please ignore this. 

I tried "add" instead of sum and it worked.

@Carl Love

Thank you 

@Christian Wolinski 

Thank you, Christian

@dharr 

Thank you so much

Hi Tomleslie here is the link to my code: NNMMM.mw

@dharr 

Thank you 

@tomleslie 

Thank you for the explanation

@tomleslie 

I'm kind of confused. Please in mydata, which column corresponds to xi =0 and xi =0.1. I want to differentiate them for plotting purposes.

@tomleslie

 Thank you very much. I really appreciate your help