cos2x=1/2,求X等于?已知cos2x=1/2,其中X的取值范围是kπ+π/2到（k+1）π,求X?我算出来是kπ+π/6,答案是kπ+5π/6.为啥呢?求详细说明,
问题描述：
cos2x=1/2,求X等于?
已知cos2x=1/2,其中X的取值范围是kπ+π/2到（k+1）π,求X?我算出来是kπ+π/6,答案是kπ+5π/6.为啥呢?求详细说明,
答
答：
cos(2x)=1/2
2x=2kπ+π/3或者2x=2kπ-π/3
所以：
x=kπ+π/6或者x=kπ-π/6=(k-1)π+5π/6=Kπ+5π/6
因为：
kπ+π/2所以：x=kπ+5π/6
答
∵kπ+π/2
则2x=2kπ+5π/3
x=kπ+5π/6
你的问题出在2x你以为是在第一象限，其实在第四象限
答
cos(2x)=1/2
2x = 2k'π ± π/3
x = k'π ± π/6
put k-1 = k'
x = (k+1)π ± π/6
x的取值范围是kπ+π/2到(k+1)π
=>x = (k+1)π - π/6 = kπ+5π/6
答
六分之π比二分之π小啊→_→