Add the following: a(a+b), b(b+c), c(a+c) and a2 + b2 + c2
a(a+b) = a2 + ab
b(b+c) = b2 + bc
c(a+c) = ac + c2
Now, a2 + ab
+ b2 + bc
+ c2 + ac
+ a2 +b2 +c2
= 2a2 + ab + 2b2 + bc + 2c2 + ac