复合函数f|f(x)等sinx, g(x)等(x-π2)^2+π3, 求f(g(x))的单调区间

已知函数f(x)=sinx,g(x)=(x-π/2)^2+π/3,求f(g(x))的单调区间
主要内容:
本文通过正弦函数的单调性知识,介绍已知由函数f(x)=sinx,g(x)=(x-π/2)^2+π/3,构成的复合函数f(g(x))的增区间和减区间。
复合函数f|f(x)等sinx, g(x)等(x-π2)^2+π3, 求f(g(x))的单调区间
文章插图

主要步骤:
1.函数的表达式。
解:对于函数g(x)有:
g(x)=(x-π/2)^2+π/3≥π/3。
复合函数f(g(x))的表达式为:
f(g(x))=sin[(x-π/2)^2+π/3]
2.复合函数的增区间
(1)当2kπ+π/3≤(x-π/2)^2+π/3≤2kπ+π/2时,
2kπ≤(x-π/2)^2≤2kπ+π/3,
2kπ≤(x-π/2)^2≤(6k+1)π/3,
1).当(x-π/2)^2≥2kπ/1时,
x-π/2≥√(2kπ)或者x-π/2≤-√(2kπ),
x≥π/2+√(2kπ)或者x≤π/2–√(2kπ),
2).当(x-π/2)^2≤(6k+1)π/3时,
-√3(6k+1)π/3≤x-π/2≤√3(6k+1)π/3,
π/2-√3(6k+1)π/3≤x≤π/2+√3(6k+1)π/3,
综上,此时复合函数的增区间为:
[π/2+√(2kπ),π/2+√3(6k+1)π/3];
[π/2–√3(6k+1)π/3,π/2–√(2kπ)],k∈Z+。
(2)当2kπ+3π/2≤(x-π/2)^2+π/3≤2kπ+2π时,
2kπ+7π/6≤(x-π/2)^2≤2kπ+5π/3,
(2k+7)π/6≤(x-π/2)^2≤(6k+5)π/3,
1).当(x-π/2)^2≥(2k+7)π/6时,
x-π/2≥√(2k+7)π或者x-π/2≤-√(2k+7)π,
x≥π/2+√(2k+7)π或者x≤π/2-√(2k+7)π.
-√3(6k+5)π/3≤x-π/2≤-√3(6k+5)π/3,
π/2–√3(6k+5)π/3≤x≤π/2+√3(6k+5)π/3,
综上,此时复合函数的增区间为:
[π/2+√(2k+7)π,π/2+√3(6k+5)π/3]
[π/2–√3(6k+5)π/3,π/2-√(2k+7)π].
复合函数f|f(x)等sinx, g(x)等(x-π2)^2+π3, 求f(g(x))的单调区间
文章插图

3.复合函数的减区间
根据正弦函数的单调减区间,有:
2kπ+π/2≤(x-π/2)^2+π/3≤2kπ+3π/2,
2kπ+π//3≤(x-π/2)^2≤2kπ+7π/6.
(6k+1)π/3≤(x-π/2)^2≤(2k+7)π/6。
(1)当(x-π/2)^2≥(6k+1)π/3时,
x-π/2≥√3(6k+1)π/3或者
x-π/2≤-√3(6k+1)π/3,
不等式移项,有:
x≥π/2+√3(6k+1)π/3或者
x≤π/2-√3(6k+1)π/3,
进一步解不等式有:
x≥π/2+√3(6k+1)π/3或者
x≤π/2–√3(6k+1)π/3.
(2)当(x-π/2)^2≤(2k+7)π/6时,
-√(2k+7)π≤x-π/2≤√(2k+7)π,
π/2–√(2k+7)π≤x≤π/2+√(2k+7)π,
此时复合函数的减区间为:
[π/2+√3(6k+1)π/3,π/2+√(2k+7)π];
【 复合函数f|f(x)等sinx, g(x)等(x-π2)^2+π3, 求f(g(x))的单调区间】[π/2–√(2k+7)π,π/2–√3(6k+1)π/3]。

    推荐阅读