Test 1 Sec4 #24 help!

For this one, you’re looking for the equation of a circle with a center of that passes through . The best way to go here is just to build the equation.

For this test, you should know the standard circle equation: A circle with center and radius *r* has the equation . You can fill in the left side immediately: The center of this circle is (0, 4), so the left side should be .

To fill in the right side, you need to find (or ). Easy enough:

So the answer you want is A.

A note for those who like quick and dirty solutions: Once you’ve got the left side of the equation, you know the answer must be either A or C. If you’re paying close attention, you’ll notice that the only difference between A and C is on the right side of the equation: A says and C says . If you’re in a hurry, that’s probably good enough—you know you’re looking for a squared value, and A provides it.

Another note: if you can’t remember the circle equation, you can still get this question right! Use your calculator to plug and into each answer choice. Only one will work. 🙂