MySQL Forums :: Newbie :: Problem with warning "data truncated for column "u" at row 1"

I have a function where I declare a variable:

DECLARE u DECIMAL(65,10);

Then I set the variable:

SET u = ((x3-x1)*(x2-x1)+(y3-y1)*(y2-y1))/(POW(x2-x1,2)+POW(y2-y1,2));

where the type of xn and yn is the same as u. I tried truncating the result:

SET u = TRUNCATE(((x3-x1)*(x2-x1)+(y3-y1)*(y2-y1))/(POW(x2-x1,2)+POW(y2-y1,2)),9);

But I still get the warning. Does anyone know what I am doing wrong?