x5+y5−(x4y+xy4)=x5−x4y+y5−xy4=x4(x−y)+y4(y−x)=(x4−y4)(x−y)=(x+y)(x−y)2(x2+y2)⩾0đpcm
$
để ......................\Rightarrow x^5 + y^5 - (x^4y + xy^4)
\geqslant 0=x5−x4y+y5−xy4= x^4(x - y) + y^4(y - x)
=(x4−y4)(x−y)= (x + y)(x - y)^2(x^2 + y^2) \geqslant 0 $$đpcm$