X²+kx+7=0
x1+x2=-k
x1×x2=7
{x1+x2=-k
{x1-x2=1
x1+x2-x1-(-x2)=-k-1
x1+x2-x1+x2=-k-1
2x2=-k-1|÷2
x2=-(k+1)/2
x1+x2+x1-x2=-k+1
2x1=1-k|÷2
x1=(1-k)/2
x1×x2=7
((1-k)/2)×(-(k+1)/2)=7|×(-2)
(1-k)×(1+k)=-14
1-k²=-14
-k²=-14-1
-k²=-15|×(-1)
k²=15
k1=-√15
k2=√15