∵x2-xy-2y2=0，∴（x-2y）（x+y）=0，∴x-2y=0或x+y=0．∴x=2y或x=-y．当x=2y时，x2−2xy−5y2x2+2xy−5y2＝(2y)2−2•2y•y−5y2(2y)2+2•2y•y+5y2＝−5y213y2＝−513；当x=-y时，x2−2xy−5y2x2+2xy+5y2＝(−y)2...