Using relations of sin and cosine to find answer question.

Cos (23pi/2) = cos ( pi/2) = 0        Sin(theta) = 0 at nowhere BETWEEN 0 and pi/2

     it DOES equal 0 AT 0        But your question stated theta>0

We know that 23pi/2 is in between 270°(3pi/2) and 360°(2pi).

 

So, it's reference angle is 24pi/12 - 23pi/2 (because 2pi/1 is equal to 24pi/12).

The reference angle will be pi/12.

 

We know that sin+cos is equal to 90. and 90° in radians is pi/2. So, (pi/2) - (pi/12) will give you the value of theta, which is 5pi/12