Răspuns :
2001²-2001-2000=
=2001*(2001-1)-2000=
=2001*2000-2000=
=2000*(2001-1)=
=2000*2000=2000²- p.p
=2001*(2001-1)-2000=
=2001*2000-2000=
=2000*(2001-1)=
=2000*2000=2000²- p.p
a=2003+2*(1+2+....+2002)=2003+2(2002*2003:2)=2003(1+2002)=2003*2003=2003² care este patrat perfect