[질문] (a+b+c)의세제곱은 어떻게풀어요

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