(-1/2)^(-2)+(-1/3)^(-2)
=1/[(-1/2)^2]+1/[(-1/3)^2]
=4+9
=13
(a-b+c)^2
=(a-b+c)(a-b+c)
=a²-ab+ac-ab+b²-bc+ac-bc+c²
=a²+b²+c²-2(ab-ac+bc)