.求y=根号下(2cosx-1)的定义域?求y=lg(3-4sin²x)的定义域?

问题描述:

.求y=根号下(2cosx-1)的定义域?
求y=lg(3-4sin²x)的定义域?

2cosx-1>=0
cosx>=0.5
x∈(π/3+2kπ,2π/3+2kπ,k∈Z)

3-4sin²x>=0
sin²xsinx∈(-根号下3/2,+根号下3/2)
x∈(2kπ,2kπ+π/3)∪(2kπ+2π/3,2kπ+4π/3)∪(2kπ+5π/3,(2k+1)π),k∈Z

y=√(2cosx-1)
2cosx-1>0
cosx>1/2 x∈(2kπ-π/3,2kπ+π/3)

y=lg(3-4sin²x)
3-4sin²x>0
cos2x>-1/2 x∈(kπ-π/6,kπ+π/6)

1、2cosx-1≧0
得:cosx≧1/2
由余弦曲线得:-π/3+2kπ≦x≦π/3+2kπ
即定义域为【-π/3+2kπ,π/3+2kπ】,k∈Z
2、3-4sin²x>0
得:-(√3)/2

1.y=根号下(2cosx-1)
2cosx-1≥0
cosx≤1/2
2kπ+ π/3≤x≤2kπ+5π/3

2、y=lg(3-4sin²x)
3-4(sinx)^2>0
(sinx)^2-√3 /22kπ- π/3