>>284

a【1】=p(実数)
a【n】=x^n+y^n+z^nとおく

a【n+1】=x^(n+1)+y^(n+1)+z^(n+1)
=(x^n+y^n+z^n)(x+y+z)-(y+z)x^(n-1)-(z+x)y^(n-1)-(x+y)z^(n-1)
=(x^n+y^n+z^n)p-p(x^(n-1)+y^(n-1)+z^(n-1))+x^(n-2)+y^(n-2)+z^(n-2)
=pa【n】-pa【n-1】+a【n-2】　(n≧3)

後は帰納法