bc=a^2-8a+7
两式相减得
a^2-2bc-b^2-c^2-2a+1=0
(b+c)^2=(a-1)^2
b+c=±a-1
b,c是方程x^2±(a-1)x+a^2-8a+7=0的两根
Δ=(a-1)^2-4(a-1)(a-7)>=0
(a-1)(a-1-4a+28)>=0
(a-1)(27-3a)>=0
1