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Write the equation of the circle with a center at ( -7,4 ) and a diameter of 18

A ) ( x + 7 ) 2 + ( y – 4 ) 2 = 18
B ) ( x - 7 ) 2 + ( y + 4 ) 2 = 324
C ) ( x + 7 ) 2 + ( y – 4 ) 2 = 324
D ) ( x + 7 ) 2 + ( y – 4 ) 2 = 81
E ) ( x – 7 ) 2 + ( y + 4 ) 2 = 81

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Answer:

(x +7)^2 + (y - 4)^2 = 81.  This matches Answer Choice D.

Step-by-step explanation:

Apply the standard equation of a circle with center at (h, k) and radius r:

(x - h)^2 + (y - k)^2 = r^2

Here (h, k) is (-7, 4) and r is 18/2 or 9, and so the desired equation is

(x +7)^2 + (y - 4)^2 = 81.  This matches Answer Choice D.

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