设f(x)=x+4x,(1)判断f(x)的奇偶性;(2)判断f(x)在(0,2]和[2,+∞)的单调性,并用定义证明.-高一数学

题目简介

设f(x)=x+4x,(1)判断f(x)的奇偶性;(2)判断f(x)在(0,2]和[2,+∞)的单调性,并用定义证明.-高一数学

题目详情

f(x)=x+
4
x

(1)判断f(x)的奇偶性;
(2)判断f(x)在(0,2]和[2,+∞)的单调性,并用定义证明.
题型:解答题难度:中档来源:不详

答案

(1)由f(x)=x+class="stub"4
x
知,定义域为{x|x≠0}
显然,定义域关于原点对称.
f(-x)=-x+class="stub"4
-x
=-(x+class="stub"4
x
)
=-f(x)
所以.f(x)为奇函数
(2)①任取x1<x2且x1,x2∈(0,2]
由题意,f(x1)-f(x2)=x1+class="stub"4
x1
-(x2+class="stub"4
x2
)

=(x1-x2)+4
x2-x1
x1x2

=(x1-x2)(1-class="stub"4
x1x2

因为x1<x2且x1,x2∈(0,2]
则x1-x2<0;
0<x1x2<4,class="stub"4
x1x2
>1
,所以1-class="stub"4
x1x2
<0
=(x1-x2)(1-class="stub"4
x1x2
)>0
故f(x1)>f(x2)
所以,f(x)在(0,2]为上的减函数.
②任取x1<x2且x1,x2∈[2,+∞)
由题意,f(x1)-f(x2)=x1+class="stub"4
x1
-(x2+class="stub"4
x2
)

=(x1-x2)+4
x2-x1
x1x2

=(x1-x2)(1-class="stub"4
x1x2

因为x1<x2且x1,x2∈[2,+∞)
则x1-x2<0;
x1x2>4,0<class="stub"4
x1x2
<1
,所以1-class="stub"4
x1x2
>0
=(x1-x2)(1-class="stub"4
x1x2
)<0
故f(x1)<f(x2)
所以,f(x)在为[2,+∞)上的增函数.
∴f(x)在(0,2]上为减函数,[2,+∞)上为增函数.

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