If a = 7, b = 5 and c = 3, then the value of a2 + b2 + c2 - ab - bc - ca is?

Answer: Option A

$$\begin{array}{rl}& a^2+b^2+c^2-ab-bc-ca\\ & =\frac{1}{2}\left[{\left(a-b\right)}^2+{\left(b-c\right)}^2+{\left(c-a\right)}^2\right]\\ & =\frac{1}{2}\left[{\left(7-5\right)}^2+{\left(5-3\right)}^2+{\left(3-7\right)}^2\right]\\ & =\frac{1}{2}\left(4+4+16\right)\\ & =\frac{24}{2}\\ & =12\end{array}$$