Answer: 
We can write the four solutions as: pi/2, 5pi/4, 3pi/2, 7pi/4
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Work Shown:

Use the unit circle or reference sheet to find that
leads to
when working with the interval [0,2pi)
Also, use of the unit circle would show that
leads to
when working with the interval [0,2pi)
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Overall, the four solutions on the interval [0, 2pi) are

To verify these solutions, plug them in one at a time into the original equation. You should end up with 0 = 0 after simplifying. Make sure your calculator is in radian mode.