Read the following SAT test question and then click on a button to select your answer.

　　A circle in the xy-plane has its center at (-100,221) and a diameter of 17. Which of the following is an equation of the circle?

　　A.(x+100)^2+(y-221)^2=7

　　B.(x+100)^2+(y-221)^2=2

　　C.(x-100)^2+(y+221)^2=7

　　D.(x-100)^2+(y+221)^2=2

　　【答案解析】

　　答案：A

　　解析：

　　The formula for a circle with a center at (h,k) and a radius of r is: (x-h)^2+(y-k)^2=r^2

　　The length of the radius is half of the length of the diameter.

　　1/2*17=8.5

　　Using the coordinates of the center and the length of the radius of this circle in the equation yields: (x-(-100))^2+(y-221)^2=8.5^2

　　In standard form, the equation of this circle is: (x+100)^2+(y-221)^2=72.25

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