(x+3y-4z)(x-3y+4z)=[x+(3y-4z)][x-(3y-4z)]=x^2-(3y-4z)^2=x^2-(9y^2-24yz+16z^2)=x^2-9y^2+24yz-16z^2。
=[x+(3y-4z)][x-(3y-4z)]=x^2-(3y-4z)^2=x^2-9y^2＋24yz-16z^2
