[질문] (x+y-z)(x-y+z)를전개하여라

(x-y+z)(x+y-z)={x-(y-z)}{x+(y-z)} >> (y-z)=A로 치환하면 (x-A)(x+A)=x제곱-A제곱=x제곱-(y-z)제곱=x제곱-(y제곱-2yz+z제곱)=x제곱-y제곱+2yz-z제곱