Σ(-x2)n = (-x2)0 + (-x2)1 + (-x2)2 + (-x2)3 + ... = 1 - x2 + x4 - x6 + ...
1/(1 + x2) = ( 1 + x2 - x2 ) / (1 + x2) = 1 - x2 / (1 + x2) = 1 - (x2 + x4 - x4 ) / (1 + x2) = 1 - ( x2(1 + x2) - x4 ) / (1 + x2) = 1 - x2 + x4 / (1 + x2) = ... = 1 - x2 + x4 - x6 + ... = Σ(-x2)n