∵x＜y＜0，
∴x-y＜0，x+y＜0．
∴

x2−2xy+y2

=

(x−y)2

=|x-y|=y-x．

x2+2xy+y2

=

(x+y)2

=|x+y|=-x-y．
∴

x2−2xy+y2

+

x2+2xy+y2

=-2x，
故选C．