1/x1² +1/x2²=(x1² +x2²)/(x2x1)²
=(x1² +x2²+2x1x2)-2x1x2/(x2x1)²
=(x1+x2)²/(x2x1)² -2/x2x1
=(-b/a)²/(c/a)² -2/(c/a)
=(b² -2ac)/c²
1/x1² +1/x2²=(x1² +x2²)/(x2x1)²
=(x1² +x2²+2x1x2)-2x1x2/(x2x1)²
=(x1+x2)²/(x2x1)² -2/x2x1
=(-b/a)²/(c/a)² -2/(c/a)
=(b² -2ac)/c²