f(x)‘=-1/2*2x+b*1/(x+2)
=-x+b/(x+2)
因x>-1,所以x+2>1
(x+2)*f(x)‘=-x(x+2)+b
f(x)=-1/2x²+bln(x+2)在(-1,正无穷)上是减函数
f(x)‘=-x+b/(x+2)在(-1,正无穷)上小于0
(x+2)*f(x)‘=-x(x+2)+b在(-1,正无穷)上小于0
x^2+2x-b在(-1,正无穷)上大于0
（x+1）^2-1-b在(-1,正无穷)上大于0
-1-b>=0
b