## Example Questions

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### Example Question #163 : Solid Geometry

What is the surface area of a cone with a radius of 4 and a height of 3?      Explanation:

Here we simply need to remember the formula for the surface area of a cone and plug in our values for the radius and height. ### Example Question #162 : Solid Geometry

The lateral area is twice as big as the base area of a cone.  If the height of the cone is 9, what is the entire surface area (base area plus lateral area)?

81π

90π

27π

54π

81π

Explanation:

Lateral Area = LA = π(r)(l) where r = radius of the base and l = slant height

LA = 2B

π(r)(l) = 2π(r2)

rl = 2r2

l = 2r From the diagram, we can see that r2 + h2 = l2.  Since h = 9 and l = 2r, some substitution yields

r2 + 92 = (2r)2

r2 + 81 = 4r2

81 = 3r2

27 = r2

B = π(r2) = 27π

LA = 2B = 2(27π) = 54π

SA = B + LA = 81π

### Example Question #1 : Cones

A right cone has a radius of 4R and a height of 3R. What is the ratio of the total surface area of the cone to the surface area of just the base?      Explanation:

We need to find total surface area of the cone and the area of the base.

The area of the base of a cone is equal to the area of a circle. The formula for the area of a circle is given below: , where r is the length of the radius.

In the case of this cone, the radius is equal to 4R, so we must replace r with 4R. To find the total area of the cone, we need the area of the base and the lateral surface area of the cone. The lateral surface area (LA) of a cone is given by the following formula: , where r is the radius and l is the slant height.

We know that r = 4R. What we need now is the slant height, which is the distance from the edge of the base of the cone to the tip.

In order to find the slant height, we need to construct a right triangle with the legs equal to the height and the radius of the cone. The slant height will be the hypotenuse of this triangle. We can use the Pythagorean Theorem to find an expression for l. According to the Pythagorean Theorem, the sum of the squares of the legs (which are 4R and 3R in this case) is equal to the square of the hypotenuse (which is the slant height). According to the Pythagorean Theorem, we can write the following equation:    Let's go back to the formula for the lateral surface area (LA).   To find the total surface area (TA), we must add the lateral area and the area of the base. The problem requires us to find the ratio of the total surface area to the area of the base. This means we must find the following ratio: We can cancel , which leaves us with 36/16.

Simplifying 36/16 gives 9/4.

### Example Question #2 : Cones

You are given a right circular cone with height . The radius is twice the length of the height. What is the volume?      Explanation:

You are given a right circular cone with height 5. The radius is twice the length of the height. What is the volume?

Height = 5cm. The radius is twice the height. , so the radius is . ### Example Question #1 : Cones In terms of , express the surface area of the above right circular cone.    None of the other choices gives the correct response. Explanation:

The surface area of a right circular cone, given its slant height and the radius of its base, can be found using the formula The slant height is shown to be 24, so setting and substituting:  ### Example Question #4 : Cones In terms of , express the surface area of the provided right circular cone.      Explanation:

The surface area of a right circular cone, given its slant height and the radius of its base, can be found using the formula The height is shown in the diagram to be 20. By the Pythagorean Theorem, Setting and solving for :   Substituting in the surface area formula: ### Example Question #1 : How To Find The Volume Of A Cone

An empty tank in the shape of a right solid circular cone has a radius of r feet and a height of h feet. The tank is filled with water at a rate of w cubic feet per second. Which of the following expressions, in terms of r, h, and w, represents the number of minutes until the tank is completely filled?

π(r2)(h)/(180w)

π(r2)(h)/(20w)

20w/(π(r2)(h))

π(r2)(h)/(60w)

180w/(π(r2)(h))

π(r2)(h)/(180w)

Explanation:

The volume of a cone is given by the formula V = (πr2)/3. In order to determine how many seconds it will take for the tank to fill, we must divide the volume by the rate of flow of the water.

time in seconds = (πr2)/(3w)

In order to convert from seconds to minutes, we must divide the number of seconds by sixty. Dividing by sixty is the same is multiplying by 1/60.

(πr2)/(3w) * (1/60) = π(r2)(h)/(180w)

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

A cone has a base radius of 13 in and a height of 6 in.  What is its volume?

1352π in3

338π in3

1014π in3

4394π in3

338π in3

Explanation:

The basic form for the volume of a cone is:

V = (1/3)πr2h

For this simple problem, we merely need to plug in our values:

V = (1/3)π13* 6 = 169 * 2π = 338π in3

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

A cone has a base circumference of 77π in and a height of 2 ft.  What is its approximate volume?

142,296π in3

71,148π in3

11,858π in3

2964.5π in3

8893.5π in3

11,858π in3

Explanation:

There are two things to be careful with here.  First, we must solve for the radius of the base. Secondly, note that the height is given in feet, not inches. Notice that all the answers are in cubic inches. Therefore, it will be easiest to convert all of our units to inches.

First, solve for the radius, recalling that C = 2πr, or, for our values 77π = 2πr. Solving for r, we get r = 77/2 or r = 38.5.

The height, in inches, is 24.

The basic form for the volume of a cone is: V = (1 / 3)πr2h

For our values this would be:

V = (1/3)π * 38.52 * 24 = 8 * 1482.25π = 11,858π in3

### Example Question #1 : Solid Geometry

What is the volume of a right cone with a diameter of 6 cm and a height of 5 cm?      The general formula is given by , where = radius and = height.  