>>799
(与式) = a^4 -2(b^2 + c^2)a^2 + (b^2 -c^2)^2
    = a^4 -2(b^2 + c^2)a^2 + (b+c)^2・(b-c)^2
    = a^4 -{(b+c)^2 + (b-c)^2}a^2 + (b+c)^2・(b-c)^2
    = {a^2 -(b+c)^2}{a^2 -(b-c)^2}
    = (a+b+c)(a-b-c)(a+b-c)(a-b+c),