Answer: Option 2. Segment TS
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Explanation:
Recall that the sine of an angle is equal to the ratio of opposite over hypotenuse.
In other words,
sin(angle) = opposite/hypotenuse
With respect to reference angle theta, the side TS is opposite this angle. It is the leg furthest from the angle. The hypotenuse is the longest side segment TO. The hypotenuse is always opposite the 90 degree angle.
For a unit circle, the radius is 1, so segment TO is 1 unit long.
Therefore,
sin(angle) = opposite/hypotenuse
sin(theta) = TS/TO
sin(theta) = TS/1
sin(theta) = TS
