[질문] (a+b+c)를 제곱한것은?

(a+b+c)^2 =(a+b+c)*(a+b+c) =(a+b+c)*a+(a+b+c)*b+(a+ b+c)*c =(a^2+ab+ca)+(ab+b^2+ bc)+(ca+b+c^2) = a^2+b^2+c^2+2ab+2bc+2ca