∵a=b+2,b=c+1
∴a-b=2 b-c=1 a-c=3
a²+b²+c²-ab-bc-ac
=(2a²+2b²+2c²-2ab-2bc-2ac)/2
=(a²-2ab+b²+b²-2bc+c²+a²-2ac+c²)
=[(a-b)²+(b-c)²+(a-c)²]/2
=(2²+1²+3²)/2
=(4+1+9)²
=14/2
=7