The area (a) of a circle is a function of its radius (r) and is given by the function a = f(r) = πr2. what is the domain of this function? a. all positive real numbers b. all real numbers c. all positive real numbers including 0 d. all real numbers excluding fractions

The domain is the set of allowed inputs. In this case, its the set of allowed numbers we can choose from to replace 'r' with. 

The answer is choice C) all positive real numbers including 0
Because we can pick any positive real number we want to replace r with. The value 0 is allowed as well since it leads to an area of 0

The answer is not "all real numbers" since negative radius values make no sense.

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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.