已知函数f(x)=sinx平方+2倍根号3sin(x+派/4)cos(x-派/4)-cosx平方-根号3.1)求函数f(x)的最小正周期和单调递减区间2)求f(x)在(-派/12,25派/36)上的值域
问题描述:
已知函数f(x)=sinx平方+2倍根号3sin(x+派/4)cos(x-派/4)-cosx平方-根号3.
1)求函数f(x)的最小正周期和单调递减区间
2)求f(x)在(-派/12,25派/36)上的值域
答
1、根据积化和差公式,
f(x)=(sinx)^2+2√3sin(x+π/4)cos(x-π/4)-(cosx)^2-√3
=-cos2x+√3[sin2x+sinπ/2)-√3
=2(√3sin2x/2-cos2x/2)
=2sin(2x-π/6),
最小正周期为2π/2=π,
2kπ+π/2kπ+π/4kπ+π/3单调递减区间为:x∈[kπ+π/3,kπ+5π/6],(k∈Z),
2、单调递增区间为:x∈[kπ-5π/6,kπ+π/3],(k∈Z),
在区间(-π/12,25π/36)内,(-π/12,π/3)
为单调递增,最大值为2,最小为-√3,
(π/3,25π/36)内单调递减,最大为1,最小为2sin(2π/9),
故f(x)在(-π/12,25π/36)区间内值域:f(x)∈(-√3,2).
答
f(x)=(sinx)^2+2√3sin(x+π/4)cos(x-π/4)-(cosx)^2-√3=-((cosx)^2-(sinx)^2)+√3(sin(x+π/4+x-π/4)+sin(x+π/4-(x-π/4))-√3=-cos(2x)+√3sin(2x)+√3-√3=√3sin(2x)-cos(2x)=2sin(2x-π/6)(1)最小正周期:2...