According to the number of elements, the number of roots of a quadratic equation with one variable can be found, and then the range of values corresponding to b can be found by using the discriminant.

Detailed steps:

Solution:

A={x|x？ +4x+b=0，b，x∈R}

There is only one element in the (1) set a, so the equation x? +4x+b=0 has only one solution, namely

Δ=4? -4b=0

16-4b=0

b=4

There is only one element in the set A, so the value of b is 4.

(2)

There are two elements in set a, so the equation x? +4x+b=0 has two solutions, namely

Δ=4? -4b & gt； 0

16-4b >0

B< four

There are two elements in the set A, so the value range of b is {b lb.