][tex]a^{2} + b^{2} + c^{2} + ab+bc+ca= \frac{1}{2} [ (a+b)^{2} + (b+c)^{2} + (c+a)^{2} ] \\ \frac{1}{2} [(a^{2} +2ab+ b^{2} +b^{2} +2bc+c^{2} + c^{2} +2ac+ a^{2} )]\\ =\frac{1}{2} (2 a^{2} +2b^{2} +2 c^{2} +2ab+2ac+2bc) \\ \frac{1}{2}*2(a^{2} + b^{2} + c^{2} + ab+bc+ca) \\ =a^{2} + b^{2} + c^{2} + ab+bc+ca [/tex]