a²+b²+c²+ab+bc-ac
=1/2(2a²+2b²+2c²+2ab+2bc-2ac)
=1/2(a²+b²+2ab+a²+c²-2ac+b²+c²+2bc)
=1/2[(a+b)²+(a-c)²+(b+c)²]
代入
原式=1/2[(2005-2006)²+(2004-2006)²+(2005-2004)²]
=1/2[1+4+1]
=1/2×6
=3