单调递减.证明:因为a<0,所以f(x)图象开口向下,由图象得出f(x)在对称轴的左边单调递增,在对称轴的右边单调递减.f(x)的对称轴是直线x=-b/2a.所以f(x)在[-b/2a,+无穷大)区间上单调递减.
