The steps for deriving the Quadratic Formula are shown. Which is the BEST choice for the missing reason? A) factor
B) simplifi
C) complete the square
D) take the square root of both sides​

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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.