How to write values of numpy array member to txt in python?

repr() will give you a representation of the object. For simple objects, such as integers and floats, this will be the literal value, but this isn't the case for everything. What you want here is to use the value of the array in string interpolation. It's also recommended to handle files with the with syntax.

with open("output.txt","w") as textf:
    for i in range(len(x1)):
        textf.write('sf{0}: -1x1_1+{1:f}x_1+{2:f}x3_1\n'.format(i+2, x2[i], x3[i]))

By adding further numbers in the format, e.g. {:5.3f} you can control the maximum width the float will take up and how many digits after the decimal point will be shown. For example,

>>> '{:10.5f}'.format(3.25)
'   3.25000'

And for filling the spaces to the left with zeros:

>>> '{:010.5f}'.format(3.25)
'0003.25000'

Finally, if you have a lot of lines to write, calling write() repeatedly could be slow. It'd be more efficient to create a string of all the lines first and then write them in one go.

lines = ['sf{0}: -1x1_1+{1:f}x_1+{2:f}x3_1'.format(i+2, x2[i], x3[i]) for i in range(len(x1))]
with open("output.txt","w") as textf:
    textf.write('\n'.join(lines))