f(x)是偶函数,f(-3/2)=f(3/2)
在【0,+∞)上是减函数 ,如 x≥3/2 ,则：f(x)≤f(3/2)
∵ x= a²+2a+5/2 = (a+1）² + 3/2 ≥ 3/2
∴ f(x)=f(a²+2a+5/2) ≤ f(3/2) = f(-3/2)