函数体200分要详细

来源:百度知道 编辑:UC知道 时间:2024/05/22 04:23:11
已知函数F(x)=2sin(x+π/4)sin(x-π/4)+根号3sin2X
1 求F(X)的最小正周期
2求F(x)去到最多值时所以X的值
=√3sin2X-cosx
=2sin(2x-π/6)
这步解释下

1.F(x)=2sin(x+π/4)sin(x-π/4)+√3sin2X
=2cos[π/2-(x+π/4)]sin(x-π/4)+√3sin2X 公式sin(π/2-a)=cosa;
=2cos(-x+π/4)sin(x-π/4)+√3sin2X
=2cos(x-π/4)sin(x-π/4)+√3sin2X 余弦函数为偶函数;
=sin2(x-π/4)+√3sin2X 倍角公式;
=sin(2x-π/2)+√3sin2X
=-sin(π/2-2x)+√3sin2X 正弦函数为奇函数;
=-cos2x+√3sin2X 公式sin(π/2-a)=cosa;
=√3sin2X-cos2x
=2*[sin2x*(√3/2)-cos2x*(1/2)] 提取公因素2;
=2*[sin2x*cos(π/6)-cos2x*sin(π/6)] 转化为三角函数
=√3sin2x-cos2x
=2( √3/2sin2x-1/2cos2x)
又知√3/2=cos30°=cosπ/6,而1/2=sin60°=sinπ/3
所以有公式可知:
√3/2sin2x-1/2cos2x)
=cosπ/6*sin2x-sinπ/3*cos2x
所以原式=2(cosπ/6*sin2x-sinπ/3*cos2x)
=2sin(2x-π/6)

先化简2sin(x+π/4)sin(x-π/4),可将其拆开:
2sin(x+π/4)sin(x-π/4)
=(sinx+cosx)(sinx-cosx)
=-cos2x

则F(x)=2sin(x+π/4)sin(x-π/4)+根号3sin2X
=√3sin2x-cos2x
=2sin(2x-π/6)

所以T=2π/2=π

所以F(x)最大为2
此时2x-π/6=2kπ+π/2
化简得
x=kπ+π/3
所以x=kπ+π/3时,F(x)最大值为2