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Mathematics>Standard Multiple Choice

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

答案和解析

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