算什么？多项式展开？(a + b - c)^2 (a - b + c)^2=a^4 - 2 a^2 b^2 + b^4 + 4 a^2 b c - 4 b^3 c - 2 a^2 c^2 + 6 b^2 c^2 - 4 b c^3 + c^4
