a的平方+(m+1)a+9
=a^2+2*[(m+1)/2]*a+[(m+1)/2]^2+9-[(m+1)/2]^2
=(a+(m+1)/2]^2+9-[(m+1)/2]^2
若上式为完全平方公式,有
9-[(m+1)/2]^2＝0
(m+1)/2=3或-3
m=5或-7