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Test
Math
Domain
Geometry and Trigonometry
Skill
Circles
Difficulty
Medium
ID: 82c8325f
Modded SAT Question Bank by Abdullah Mallik

A circle in the xy-plane has its center at -4,5 and the point -8,8 lies on the circle. Which equation represents this circle?

  1. x-42+y+52=5

  2. x+42+y-52=5

  3. x-42+y+52=25

  4. x+42+y-52=25


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Correct Answer: D
Rationale

Choice D is correct. A circle in the xy-plane can be represented by an equation of the form x-h2+y-k2=r2, where h,k is the center of the circle and r is the length of a radius of the circle. It's given that the circle has its center at -4,5. Therefore, h=-4 and k=5. Substituting -4 for h and 5 for k in the equation x-h2+y-k2=r2 yields x--42+y-52=r2, or x+42+y-52=r2. It's also given that the point -8,8 lies on the circle. Substituting -8 for x and 8 for y in the equation x+42+y-52=r2 yields -8+42+8-52=r2, or -42+32=r2, which is equivalent to 16+9=r2, or 25=r2. Substituting 25 for r2 in the equation x+42+y-52=r2 yields x+42+y-52=25. Thus, the equation x+42+y-52=25 represents the circle.

Choice A is incorrect. The circle represented by this equation has its center at 4,-5, not -4,5, and the point -8,8 doesn't lie on the circle.

Choice B is incorrect. The point -8,8 doesn't lie on the circle represented by this equation.

Choice C is incorrect. The circle represented by this equation has its center at 4,-5, not -4,5, and the point -8,8 doesn't lie on the circle.

Question Difficulty: Medium
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