`a)sin(2x+\pi/6)=0`
`<=>2x+\pi/6=k\pi`
`<=>x=-\pi/12+k\pi/2` `(k in ZZ)`
Mà `x in (-\pi/3;\pi/6)`
`=>-\pi/3 < -\pi/12+k\pi/2 < \pi/6`
`<=>-0,5 < k < 0,5` `,` ` k in ZZ`
`=>k=0=>x=-\pi/12`
Vậy `x=-\pi/12` trên `(-\pi/3;\pi/6)`
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`b)cos` `x/2=\sqrt{3}/2`
`<=>x/2=+-\pi/6+k2\pi`
`<=>x=+-\pi/3+k4\pi`
`@x=\pi/3+k4\pi` Mà `x in (2\pi;3\pi)`
`=>2\pi < \pi/3+k4\pi < 3\pi`
`<=>5/12 < k < 2/3` `,` `k in ZZ`
`=>k=\emptyset`
`@x=-\pi/3+k4\pi` Mà `x in (2\pi;3\pi)`
`=>2\pi < -\pi/3+k4\pi < 3\pi`
`<=>7/12 < k < 5/6`
`=>k=\emptyset`
Vậy ptr vô nghiệm trên `(2\pi;3\pi)`
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