SSAT Upper Level Math : Circles

Study concepts, example questions & explanations for SSAT Upper Level Math

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Example Questions

Example Question #311 : Geometry

A square on the coordinate plane has as its vertices the points \displaystyle (0,0), (0, 8), (8,0),(8,8). Give the equation of a circle inscribed in the square.

Possible Answers:

\displaystyle (x-4)^{2}+(y-4)^{2} = 16

\displaystyle (x-8)^{2}+(y-8)^{2} = 32

\displaystyle (x-8)^{2}+(y-8)^{2} = 16

\displaystyle (x-8)^{2}+(y-8)^{2} = 64

\displaystyle (x-4)^{2}+(y-4)^{2} = 32

Correct answer:

\displaystyle (x-4)^{2}+(y-4)^{2} = 16

Explanation:

Below is the figure with the circle and square in question:

Circle on axes

The center of the inscribed circle coincides with that of the square, which is the point \displaystyle (4,4). 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, 

\displaystyle (x-h)^{2}+(y-k)^{2} = r^{2},

substitute \displaystyle h = k = 4 and \displaystyle r = 4.

\displaystyle (x-4)^{2}+(y-4)^{2} = 4^{2}

\displaystyle (x-4)^{2}+(y-4)^{2} = 16

Example Question #252 : Coordinate Geometry

Give the area of the circle on the coordinate plane whose equation is

\displaystyle x^{2}+ y^{2}+ 4x- 8y -11 = 0.

Possible Answers:

\displaystyle 21 \pi

\displaystyle 20 \pi

\displaystyle 18 \pi

\displaystyle 31 \pi

\displaystyle 11 \pi

Correct answer:

\displaystyle 31 \pi

Explanation:

The standard form of the equation of a circle is 

\displaystyle (x-h)^{2}+(y-k)^{2} = r^{2}

where \displaystyle r 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:

\displaystyle x^{2}+ y^{2}+ 4x- 8y -11 = 0

\displaystyle x^{2}+ y^{2}+ 4x- 8y =11

\displaystyle \left (x^{2}+ 4x \right ) +\left ( y^{2}- 8y \right ) =11

Complete the squares. Since \displaystyle \left ( \frac{4}{2} \right )^{2} = 4 and \displaystyle \left ( \frac{-8}{2} \right )^{2} = 16, we do this as follows:

\displaystyle \left (x^{2}+ 4x + 4 \right ) +\left ( y^{2}- 8y + 16\right ) =11 + 4 + 16

\displaystyle (x-2)^{2}+(y-4)^{2} = 31

\displaystyle r^{2}= 31, and the area of the circle is 

\displaystyle A = \pi r^{2} = 31 \pi

 

Example Question #311 : Geometry

Which of the following is the equation of a circle with center at the origin and area \displaystyle 64 \pi ?

Possible Answers:

\displaystyle x^{2}+ y^{2} = 8

\displaystyle x^{2}+ y^{2} = 64

\displaystyle x^{2}+ y^{2} = 128

\displaystyle x^{2}+ y^{2} = 16

\displaystyle x^{2}+ y^{2} = 32

Correct answer:

\displaystyle x^{2}+ y^{2} = 64

Explanation:

The standard form of the equation of a circle is 

\displaystyle (x-h)^{2}+(y-k)^{2} = r^{2},

where the center is \displaystyle (h,k) and the radius is \displaystyle r.

The center of the circle is the origin, so \displaystyle h = k = 0, and the equation is 

\displaystyle x^{2}+ y^{2} = r^{2}

for some \displaystyle r.

The area of the circle is \displaystyle 64 \pi, so

\displaystyle A = \pi r^{2} = 50 \pi

\displaystyle \frac{ \pi r^{2}}{\pi} = \frac{ 64 \pi}{\pi}

\displaystyle r^{2} = 64

We need go no further; we can substitute to get the equation \displaystyle x^{2}+ y^{2} = 64.

Example Question #531 : Ssat Upper Level Quantitative (Math)

Which of the following is the equation of a circle with center at the origin and circumference \displaystyle 64 \pi ?

Possible Answers:

None of the other responses gives the correct answer.

\displaystyle x^{2}+ y^{2} = 32

\displaystyle x^{2}+ y^{2} = 16

\displaystyle x^{2}+ y^{2} = 128

\displaystyle x^{2}+ y^{2} = 64

Correct answer:

None of the other responses gives the correct answer.

Explanation:

The standard form of the equation of a circle is 

\displaystyle (x-h)^{2}+(y-k)^{2} = r^{2},

where the center is \displaystyle (h,k) and the radius is \displaystyle r

 

The center of the circle is the origin, so \displaystyle h = k = 0.

The equation will be

\displaystyle x^{2}+ y^{2} = r^{2}

for some \displaystyle r.

The circumference of the circle is \displaystyle 64 \pi, so

\displaystyle C = 2 \pi r = 64 \pi

\displaystyle \frac{2 \pi r }{2 \pi }= \frac{64 \pi}{2 \pi }

\displaystyle r = 32

\displaystyle r^{2}= 32^{2} = 1,024

The equation is \displaystyle x^{2}+ y^{2} = 1,024, which is not among the responses.

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