由已知两式相加得a-c=8
a²+b²+c²-ab-bc-ac
=1/2(2a²+2b²+2c²-2ab-2bc-2ac)
=1/2[(a-b)²+(b-c)²+(a-c)²]
=1/2(5²+3²+8²)
=1/2×100
=50