y'=x^2+2bx+(b+2)
若函数y=1/3x^3+bx^2+(b+2)x+3有三个单调区间则方程y'=x^2+2bx+(b+2)=0有两个不相等的实根
△=4b^2-4(b+2)>0
b>2或b<-1