# SSAT Upper Level Math : How to find the equation of a circle

## Example Questions

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### Example Question #11 : How To Find The Equation Of A Circle

A square on the coordinate plane has as its vertices the points . Give the equation of a circle inscribed in the square.      Explanation:

Below is the figure with the circle and square in question: The center of the inscribed circle coincides with that of the square, which is the point . Its diameter is equal to the sidelength of the square, which is 8, so, consequently, its radius is half this, or 4. Therefore, in the standard form of the equation, ,

substitute and .  ### Example Question #12 : How To Find The Equation Of A Circle

Give the area of the circle on the coordinate plane whose equation is .      Explanation:

The standard form of the equation of a circle is where is the radius of the circle.

We can rewrite the equation we are given, which is in general form, in this standard form as follows:   Complete the squares. Since and , we do this as follows:   , and the area of the circle is ### Example Question #13 : How To Find The Equation Of A Circle

Which of the following is the equation of a circle with center at the origin and area ?      Explanation:

The standard form of the equation of a circle is ,

where the center is and the radius is .

The center of the circle is the origin, so , and the equation is for some .

The area of the circle is , so   We need go no further; we can substitute to get the equation .

### Example Question #14 : How To Find The Equation Of A Circle

Which of the following is the equation of a circle with center at the origin and circumference ?  None of the other responses gives the correct answer.  None of the other responses gives the correct answer.

Explanation:

The standard form of the equation of a circle is ,

where the center is and the radius is The center of the circle is the origin, so .

The equation will be for some .

The circumference of the circle is , so    The equation is , which is not among the responses.

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