ACT Math : Geometry

Study concepts, example questions & explanations for ACT Math

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Example Questions

Example Question #781 : Geometry

The surface area of a cone equals \(\displaystyle 24\pi \textup{cm}^2.\)If the radius of the cone equals \(\displaystyle 3\textup{cm}\), what is the height of the cone?

Possible Answers:

\(\displaystyle 4\textup{cm}\)

\(\displaystyle 3\textup{cm}\)

\(\displaystyle 6\textup{cm}\)

\(\displaystyle 8\textup{cm}\)

\(\displaystyle 5\textup{cm}\)

Correct answer:

\(\displaystyle 4\textup{cm}\)

Explanation:

To solve this question, you need to know the surface area of a cone equation:  \(\displaystyle \textup{surface area}=\pi r(r+\sqrt{h^2 +r^2}),\) where \(\displaystyle r\) is the radius of the cone \(\displaystyle (3\textup{cm})\), and \(\displaystyle h\) is the height of the cone.  We must substitute all of the values that we know, and solve for the height of the cone to get the answer.   

\(\displaystyle 24\textup{cm}^2=\pi *3\textup{cm}(3\textup{cm}+\sqrt{h^2 +(3\textup{cm})^2}),\)  Therefore, the height of the cone is \(\displaystyle 4\textup{cm.}\)

Example Question #3 : Solid Geometry

You have an empty cylinder with a base diameter of 6 and a height of 10 and you have a cone full of water with a base radius of 3 and a height of 10.  If you empty the cone of water into the cylinder, how much volume is left empty in the cylinder?

Possible Answers:

\(\displaystyle 45\pi\)

\(\displaystyle 30\pi\)

\(\displaystyle 60\pi\)

\(\displaystyle 120\pi\)

\(\displaystyle 90\pi\)

Correct answer:

\(\displaystyle 60\pi\)

Explanation:

Cylinder Volume = \(\displaystyle \pi*r^{2}*h\)

Cone Volume = \(\displaystyle \frac{1}{3}*\pi*r^{2}*h\)

Cylinder Diameter = 6, therefore Cylinder Radius = 3

Cone Radius = 3

Empty Volume = Cylinder Volume – Cone Volume

\(\displaystyle =\pi*r^{2}*h-\frac{1}{3}*\pi*r^{2}*h\) 

\(\displaystyle =\pi*3^{2}*10-\frac{1}{3}*\pi*3^{2}*10\)

\(\displaystyle =\pi*90-\pi*30\)

\(\displaystyle =\pi*60\)

Example Question #781 : Geometry

You have two cones, one with a diameter of \(\displaystyle 6\) and a height of \(\displaystyle 10\) and another with a diameter of \(\displaystyle 3\) and a height of \(\displaystyle 8\). If you fill the smaller cone with sand and dump that sand into the larger cone, how much empty space will be left in the larger cone?

Possible Answers:

\(\displaystyle 12\pi\)

\(\displaystyle 8\pi\)

\(\displaystyle 24\pi\)

\(\displaystyle 48\pi\)

\(\displaystyle 6\pi\)

Correct answer:

\(\displaystyle 24\pi\)

Explanation:

1. Find the volume of each cone:

\(\displaystyle Cone Volume=\frac{1}{3}\pi r^{2}h\)

Cone 1:

Since the diameter is \(\displaystyle 6\), the radius is \(\displaystyle 3\).

\(\displaystyle Volume=\frac{1}{3}\pi (3^{2})(10)=30\pi\)

 

Cone 2:

Since the diameter is \(\displaystyle 3\), the radius is \(\displaystyle 1.5\).

\(\displaystyle Volume=\frac{1}{3}\pi (1.5^{2})(8)=6\pi\)

 

2. Subtract the smaller volume from the larger volume:

\(\displaystyle 30\pi-6\pi=24\pi\)

Example Question #1 : How To Find The Volume Of A Cone

You have an empty cone and a cylinder filled with water. The cone has a diameter of \(\displaystyle 8\) and a height of \(\displaystyle 9\). The cylinder has a diameter of \(\displaystyle 10\) and a height of \(\displaystyle 12\). If you dump the water from the cylinder into the cone until it is filled, what volume of water will remain in the cylinder?

Possible Answers:

\(\displaystyle 1176\pi\)

\(\displaystyle 300\pi\)

\(\displaystyle 276\pi\)

\(\displaystyle 288\pi\)

\(\displaystyle 252\pi\)

Correct answer:

\(\displaystyle 252\pi\)

Explanation:

1. Find the volumes of the cone and cylinder:

Cone:

Since the diameter is \(\displaystyle 8\), the radius is \(\displaystyle 4\).

\(\displaystyle Volume=\frac{1}{3}\pi r^{2}h\)

\(\displaystyle Volume=\frac{1}{3}\pi (4^{2})(9)=48\pi\)

Cylinder:

Since the diameter is \(\displaystyle 10\), the radius is \(\displaystyle 5\).

\(\displaystyle Volume=\pi r^{2}h\)

\(\displaystyle Volume=\pi (5^{2})(12)=300\pi\)

 

2. Subtract the cone's volume from the cylinder's volume:

\(\displaystyle 300\pi-48\pi=252\pi\)

Example Question #3 : How To Find The Volume Of A Cone

What is the volume of a cone with a height of 7 cm and a radius of 4 cm? Leave your answer in terms of \(\displaystyle \pi\) and as a fraction if need be.

Possible Answers:

\(\displaystyle \frac{448}{3}\pi cm^3\)

\(\displaystyle \frac{56}{3} \pi cm^3\)

\(\displaystyle \frac{112}{3}\pi cm^3\)

\(\displaystyle 112\pi cm^3\)

\(\displaystyle \frac{196}{3}\pi cm^3\)

Correct answer:

\(\displaystyle \frac{112}{3}\pi cm^3\)

Explanation:

To find the volume of a cone plug the radius and height into the formula for the volume of a cone.

\(\displaystyle V = \frac{1}{3}\pi r^2\cdot h\)

\(\displaystyle \\=\frac{1}{3}\pi 4^2\cdot 7\\ \\=\frac{112}{3}\pi cm^3\)

Example Question #3 : How To Find The Volume Of A Cone

Which of the following will quadruple the volume of a cone?

  1. Doubling the radius
  2. Doubling the height
  3. Quadrupling the height
Possible Answers:

2 only

1, 2 and 3

1 and 3

1 only

3 only

Correct answer:

1 and 3

Explanation:

Bearing in mind the volume formula for a cone:

\(\displaystyle V_{cone} = \frac{1}{3}\pi r^2h\)

Because the volume varies by the square of the radius, doubling the radius will quadruple the volume (since \(\displaystyle 2^2 = 4\).) Because the volume also varies linearly by the height, quadrupling the height will quadruple the volume.

Example Question #3 : How To Find The Volume Of A Cone

What is the volume of a cone with a radius of \(\displaystyle 6\) and a height of \(\displaystyle 2\)? Leave your answer in terms of \(\displaystyle \pi\), reduce all fractions.

Possible Answers:

\(\displaystyle 72\pi\)

\(\displaystyle 8\pi\)

\(\displaystyle 231.32\pi\)

\(\displaystyle 24\pi\)

\(\displaystyle 36\pi\)

Correct answer:

\(\displaystyle 24\pi\)

Explanation:

To find the volume of a cone with radius \(\displaystyle r\), and height \(\displaystyle h\) use the formula:

\(\displaystyle V = \frac{1}{3}\pi *r^2*h\).

We plug in our given radius and height to find:

\(\displaystyle V = \frac{1}{3}\pi * 6^2*2 = \frac{72}{3}\pi = 24\pi\)

Example Question #2 : How To Find The Volume Of A Cone

A conical paper cup is being used as a makeshift funnel for motor oil. If the cup is 100 millimeters deep at the center and has a radius of 70 millimeters, how many cubic millimeters of motor oil can it hold at one time? Round your final answer to the nearest integer.

Possible Answers:

\(\displaystyle 233333\pi mm^2\)

\(\displaystyle 106667\pi mm^2\)

\(\displaystyle 99999\pi mm^2\)

\(\displaystyle 784333\pi mm^2\)

\(\displaystyle 387667\pi mm^2\)

Correct answer:

\(\displaystyle 233333\pi mm^2\)

Explanation:

The formula for the volume of a cone is:

\(\displaystyle V = \frac{\pi r^2h}{3}\)

With the information we have, we can simply plug values into this equation and solve.

\(\displaystyle V = \frac{\pi r^2h}{3} = \frac{(100^2)(70)\pi}{3} \approx 233333\pi\)

So our cone can hold approximately \(\displaystyle 233333\pi\) cubic millimeters of motor oil.

Example Question #2 : How To Find The Volume Of A Cone

Find the volume of a cone whose diameter is \(\displaystyle 6\) and height is \(\displaystyle 2\).

Possible Answers:

\(\displaystyle 18\pi\)

\(\displaystyle 6\pi\)

\(\displaystyle 36\pi\)

\(\displaystyle 72\pi\)

\(\displaystyle 72\pi\)

Correct answer:

\(\displaystyle 6\pi\)

Explanation:

To find the volume of a cone, simply use the following formula. Thus,

\(\displaystyle V=\frac{1}{3}\pi{r^2}h=\frac{1}{3}*\pi*3^2*2=\frac{1}{3}*9*2*\pi=6\pi\)

Example Question #5 : How To Find The Volume Of A Cone

Find the volume of a cone with radius \(\displaystyle 3\) and height of \(\displaystyle 4\).

Possible Answers:

\(\displaystyle 12\pi\)

\(\displaystyle 36\pi\)

\(\displaystyle 108\pi\)

\(\displaystyle 28\pi\)

Correct answer:

\(\displaystyle 12\pi\)

Explanation:

To solve simply use the formula for the voluma of a cone. Thus,

\(\displaystyle V=\frac{1}{3}\pi{r^2}h=\frac{1}{3}*\pi*3^2*4=12\pi\)

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