x1x2n+x2x(2n-1)+...+xnx(n+1) = (y1+y2n)(y1-y2n) + (y2+y2n-1)(y2-y2n-1) + ...+ (yn+yn+1)(yn-yn+1) = y1^2-y2n^2 + y2^2-y(2n-1)^2 + ... + yn^2-y(n-1)^2 符号差为 n-n=0.
