设方程x^2 +kx+45=0的两个实数根为x1、x2
x1+x2==-k
x1x2=45
(x1-x2)^2=16
x1^2-2x1x2+x2^2=16
x1^2+2x1x2+x2^2-4x1x2=16
(x1+x2)^2-4x1x2=16
k^2-4*45=16
k^2=196
k=14 或k=-14