x1,x2是方程ax^2+bx+c=0(a≠0)的两根
x1+x2=-b/a,x1x2=c/a
1)
x1^2+x2^2=(x1+x2)^2-2x1x2
=b^2/a^2-2c/a
=(b^2-2ac)/a^2
2)
1/x1^2+1/x2^2+b/c
=(x1^2+x2^2)/x1^2x2^2+b/c
=[(b^2-2ac)/a^2]/[c^2/a^2]+b/c
=(b^2-2ac)/c^2+b/c
=(b^2-2ac+bc)/c^2