Tan α= 21/20, α lies in quadrant III, and cos β = -5/13, β lies in quadrant II, Find sin ( α + β ) ?

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since the cosine is the ratio of the adjacent side to the hypotenuse, you can see that cos 60° = ½. answer. according to the property of cofunctions (topic 3), sin 30° is equal to cos 60°. sin 30° = ½.