Krishna
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Step 1: Know about the unit circle

      NOTE: Any point on the unit circle will be a distance of one unit from the center

                  this is the definition of the unit circle.

  • The terminal side of an angle \theta in standard position intersects the unit circle at   \cos \theta, \sin \theta.
  • Another thing you can see from the unit circle is that the values of sine and cosine will never be more than 1 or less than –1.


Step 2: Use the unit circle properties to find the trigonometric ratio.

            NOTE:  The terminal side of an angle \theta in standard position intersects the  

            unit circle at \cos \theta, \sin \theta  

                      


              Since the terminal side of  \theta intersects the unit circle

                at (\frac{84}{85}, \frac{13}{85}) = (\cos \theta, \sin \theta)

                Therefore, \sin \theta = \frac{13}{85}