集合A={x|x^2+(a+1)x+b=0}中仅一个元素a,
x^2+(a+1)x+b=0仅一个解a
△=(a+1)^2-4b=0
b=(a+1)^2/4
a^2+(a+1)a+b=0
a^2+(a+1)a+(a+1)^2/4=0
9a^2+6a+1=0
(3a+1)^2=0
a=-1/3