Từ đề bài => x7y+xy7+x5y3+x3y5+x2y6+x6y2+x4y4+y8=4(x4+y4).(x2+y2).(x+y).(x-y)
=>xy(x2+y2)(x4-x2y2+y4)+x3y3(x2+y2)+x2y2(x4+y4)+y4(x2+y2)=4(x4+y4).(x2+y2).(x+y).(x-y)
=>xy(x2+y2)(x4+y4)+y2(x2+y2)(x4+y4)=4(x4+y4).(x2+y2).(x+y).(x-y)
=>xy+y2=4.(x-y)(x+y)
=>y=4.(x-y)
=>5y=4x