Question

若函數(shù)f(x)=lg［x ²+2(a-6)x+40］在(-∞,4)上是減函數(shù),則實(shí)數(shù)a的取值范圍是(　　)

A. (-1, 2］

B. (-∞, 2)

C. (-∞, 10)

D. ［-1, 10］

Answer 1

解析:由復(fù)合函數(shù)單調(diào)性規(guī)律,可判斷真數(shù)對(duì)應(yīng)的二次函數(shù)對(duì)稱軸小于等于4,并且真數(shù)大于零,即當(dāng)自變量取4時(shí)x ²+2(a-6)x+40＞0,從而解出參數(shù).

答案:A