Start by drawing it!

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.

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.

from Tumblr https://ift.tt/2zgoxIQ