Start by drawing it!
![](https://i0.wp.com/66.media.tumblr.com/9f5c927fa56c96b76542280c316d6d65/tumblr_inline_phhtdpQmYx1qio24w_540.png?w=852&ssl=1)
Note that OC = 5 and OD = 5 because both of those are also radii. Note also that because chord CD is perpendicular to OB, it’s bisected by OB. In other words, it’s split into 2 segments each measuring 4.
![](https://i0.wp.com/66.media.tumblr.com/3fdf5ab2d2fac4ec86c752fd0e13e2c3/tumblr_inline_phhtk3dqFx1qio24w_540.png?w=852&ssl=1)
Things are really coming together! Because we know our Pythagorean triples when we see them, we know that we’ve got 3-4-5 triangles here. Therefore, the sine of angle OCD is 3/5.
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