与式=x^3+y^3×(-1)^3-3xy(-1)
={x+y+(-1)}{x^2+y^2+(-1)^2-xy-y(-1)-(-1)x}
=(x+y-1)(x^2+y^2-xy+x+y+1)
※ 公式 a^3+b^3+c^3-3abc
=(a+b+c)(a^2+b^2+c^2-ab-bc-ca)
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