CÂu 5:

`sinx=1/2`

`<=> sinx=sin([pi]/6)`

`<=> x=[pi]/6+k2pi` hoặc `x=[5pi]/6+k2pi(k in ZZ)`

Vì `0<=x<=4pi`

Xét `x=[pi]/6+k2pi`

`0<=[pi]/6+k2pi<=4pi`

`<=> -[pi]/6<=k2pi<=[23pi]/6`

`<=> -1/12<=k<=23/12`

`-> k in {0;1} -> x=[pi]/6`, `x=[13pi]/6`

Xét `x=[5pi]/6+k2pi`

`0<=[5pi]/6+k2pi<=4pi`

`<=> -[5pi]/6<=k2pi<=[19pi]/6`

`<=> -5/12<=k<=19/12`

`-> k in {0;1} -> x=[5pi]/6`, `x=[17pi]/6`

Tổng: `T=[pi]/6+[13pi]/6+[5pi]/6+[17pi]/6=6pi`

`-> bbA`