$a, (a+b)^2=(a-b)^2+4ab$
$VT=a^2+2ab+b^2$
$=a^2-2ab+4ab+b^2$
$=(a-b)^2+4ab=VP(đpcm)$
$b,(a-b)^2=(a+b)^2-4ab$
$VT=a^2-2ab+b^2$
$=a^2+2ab-4ab+b^2$
$=(a+b)^2-4ab$
$c,(a^2+b^2)(x^2+y^2)=(ax-by)^2+(ay+bx)^2$
$VT=a^2x^2+a^2y^2+b^2x^2+b^2y^2$
$VT=a^2x^2-2axby+b^2y^2+a^2y^2+2axby+b^2x^2$
$VT=(ax-by)^2+(ay+bx)^2=VP(đpcm)$
