1.
If x<1|x| < 1, y<1|y| < 1 and x1x \ne 1, then the sum to infinity of the following series (x+y)+(x2+xy+y2)+(x3+x2y+xy2+y3)+(x + y) + (x^2 + xy + y^2) + (x^3 + x^2 y + xy^2 + y^3) + \cdots is