ACT Math : Geometry

Study concepts, example questions & explanations for ACT Math

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Example Questions

Example Question #1 : How To Find The Length Of The Side Of A Right Triangle

Given a right triangle with a leg length of 2 and a hypotenuse length of √8, find the length of the other leg, x.

Vt_triangle_x-2-sqrt8

Possible Answers:

10

√8

4

6

2

Correct answer:

2

Explanation:

Using Pythagorean Theorem, we can solve for the length of leg x:

x2 + 22 = (√8)2 = 8

Now we solve for x:

x2 + 4 = 8

x2 = 8 – 4

x2 = 4

x = 2

Example Question #1 : How To Find The Length Of The Side Of A Right Triangle

The legs of a right triangle are 8\ cm\(\displaystyle 8\ cm\) and 11\ cm\(\displaystyle 11\ cm\). Rounded to the nearest whole number, what is the length of the hypotenuse?

Possible Answers:

14\ cm\(\displaystyle 14\ cm\)

2\ cm\(\displaystyle 2\ cm\)

15\ cm\(\displaystyle 15\ cm\)

9\ cm\(\displaystyle 9\ cm\)

10\ cm\(\displaystyle 10\ cm\)

Correct answer:

14\ cm\(\displaystyle 14\ cm\)

Explanation:

Use the Pythagorean Theorem. The sum of both legs squared equals the hypotenuse squared. 

Example Question #141 : Geometry

Triangles

Points \dpi{100} \small A\(\displaystyle \dpi{100} \small A\), \dpi{100} \small B\(\displaystyle \dpi{100} \small B\), and \dpi{100} \small C\(\displaystyle \dpi{100} \small C\) are collinear (they lie along the same line). \(\displaystyle \angle ACD = 90^{\circ}\)\(\displaystyle \angle CAD=30^{\circ}\), \(\displaystyle \angle CBD=60^{\circ}\), \(\displaystyle \overline{AD}=4\)

Find the length of segment \overline{BD}\(\displaystyle \overline{BD}\).

Possible Answers:

2\(\displaystyle 2\)

2\sqrt{3}\(\displaystyle 2\sqrt{3}\)

\frac{\sqrt{3}}{2}\(\displaystyle \frac{\sqrt{3}}{2}\)

\frac{4\sqrt{3}}{3}\(\displaystyle \frac{4\sqrt{3}}{3}\)

\frac{2\sqrt{3}}{3}\(\displaystyle \frac{2\sqrt{3}}{3}\)

Correct answer:

\frac{4\sqrt{3}}{3}\(\displaystyle \frac{4\sqrt{3}}{3}\)

Explanation:

The length of segment \overline{BD}\(\displaystyle \overline{BD}\) is \frac{4\sqrt{3}}{3}\(\displaystyle \frac{4\sqrt{3}}{3}\)

Note that triangles \dpi{100} \small ACD\(\displaystyle \dpi{100} \small ACD\) and \dpi{100} \small BCD\(\displaystyle \dpi{100} \small BCD\) are both special, 30-60-90 right triangles. Looking specifically at triangle \dpi{100} \small ACD\(\displaystyle \dpi{100} \small ACD\), because we know that segment \overline{AD}\(\displaystyle \overline{AD}\) has a length of 4, we can determine that the length of segment \overline{CD}\(\displaystyle \overline{CD}\) is 2 using what we know about special right triangles. Then, looking at triangle \dpi{100} \small BCD\(\displaystyle \dpi{100} \small BCD\) now, we can use the same rules to determine that segment \overline{BD}\(\displaystyle \overline{BD}\) has a length of \frac{4}{\sqrt{3}}\(\displaystyle \frac{4}{\sqrt{3}}\)

which simplifies to \frac{4\sqrt{3}}{3}\(\displaystyle \frac{4\sqrt{3}}{3}\).

Example Question #3 : How To Find The Length Of The Side Of A Right Triangle

A handicap ramp is \(\displaystyle 32\textup{ feet}\) long, and a person traveling the length of the ramp goes up \(\displaystyle 4\textup{ feet}\) vertically. What horizontal distance does the ramp cover?

Possible Answers:

\(\displaystyle 13\sqrt4\textup{ feet}\)

\(\displaystyle 12\sqrt7\textup{ feet}\)

\(\displaystyle 13\sqrt9\textup{ feet}\)

\(\displaystyle 27\textup{ feet}\)

\(\displaystyle 5\sqrt13\textup{ feet}\)

Correct answer:

\(\displaystyle 12\sqrt7\textup{ feet}\)

Explanation:

In this case, we are already given the length of the hypotenuse of the right triangle, but the Pythagorean formula still helps us. Plug and play, remembering that \(\displaystyle c\) must always be the hypotenuse:

\(\displaystyle a^2 + b^2 = c^2\) State the theorem.

\(\displaystyle a^2 = 32^2 - 4^2\) Substitute your variables.

\(\displaystyle a^2 = 1008\) Simplify.

\(\displaystyle a = \sqrt{1008} = \sqrt{144\cdot 7} = 12\sqrt7\)

Thus, the ramp covers \(\displaystyle 12\sqrt7\textup{ feet}\) of horizontal distance.

Example Question #1 : How To Find If Right Triangles Are Similar

You have two right triangles that are similar.  The base of the first is 6 and the height is 9.  If the base of the second triangle is 20, what is the height of the second triangle?

Possible Answers:

30

23

25

33

35

Correct answer:

30

Explanation:

Similar triangles are proportional.

Base1 / Height1 = Base2 / Height2

6 / 9 = 20 / Height2

Cross multiply  and solve for Height2

6 / 9 = 20 / Height2

6 * Height2=  20 * 9

Height2=  30

Example Question #1 : Right Triangles

A right triangle is defined by the points (1, 1), (1, 5), and (4, 1).  The triangle's sides are enlarged by a factor of 3 to form a new triangle.  What is the area of the new triangle?

Possible Answers:

36 square units

None of the answers are correct

81 square units

54 square units

108 square units

Correct answer:

54 square units

Explanation:

The points define a 3-4-5 right triangle.  Its area is A = 1/2bh = ½(3)(4) = 6.  The scale factor (SF) of the new triangle is 3.  The area of the new triangle is given by Anew = (SF)2 x (Aold) =

32 x 6 = 9 x 6 = 54 square units (since the units are not given in the original problem).

NOTE:  For a volume problem:  Vnew = (SF)3 x (Vold).

Example Question #86 : Right Triangles

On a flat street, a light pole 36 feet tall casts a shadow that is 9 feet long. At the same time of day, a nearby light pole casts a shadow that is 6 feet long. How many feet tall is the second light pole?

Possible Answers:

\(\displaystyle 24\)

\(\displaystyle 30\)

\(\displaystyle 18\)

\(\displaystyle 12\)

\(\displaystyle 9\)

Correct answer:

\(\displaystyle 24\)

Explanation:

Start by drawing out the light poles and their shadows.

 

Untitled

 

In this case, we end up with two similar triangles. We know that these are similar triangles because the question tells us that these poles are on a flat surface, meaning angle B and angle E are both right angles. Then, because the question states that the shadow cast by both poles are at the same time of day, we know that angles C and F are equivalent. As a result, angles A and D must also be equivalent.

Since these are similar triangles, we can set up proportions for the corresponding sides.

 

\(\displaystyle \frac{36}{x}=\frac{9}{6}\)

 

Now, solve for \(\displaystyle x\) by cross-multiplying.

\(\displaystyle 9x=216\)

\(\displaystyle x=24\)

Example Question #142 : Geometry

What is the perimeter of an equilateral triangle with an area of \(\displaystyle 56.25\sqrt{3}\)?

Possible Answers:

\(\displaystyle 22\sqrt{3}\)

\(\displaystyle 50\sqrt{3}\)

\(\displaystyle 45\sqrt{3}\)

\(\displaystyle 15\)

\(\displaystyle 45\)

Correct answer:

\(\displaystyle 45\)

Explanation:

Recall that from any vertex of an equilateral triangle, you can drop a height that is a bisector of that vertex as well as a bisector of the correlative side. This gives you the following figure:

Equigen

Notice that the small triangles within the larger triangle are both \(\displaystyle 30-60-90\) triangles. Therefore, you can create a ratio to help you find \(\displaystyle h\).

The ratio of the small base to the height is the same as \(\displaystyle 1:\sqrt{3}\).  Therefore, you can write the following equation:

\(\displaystyle \frac{0.5b}{h}=\frac{1}{\sqrt{3}}\)

This means that \(\displaystyle 0.5b\sqrt{3}=h\).

Now, the area of a triangle can be written:

\(\displaystyle A=\frac{1}{2}bh\), and based on our data, we can replace \(\displaystyle h\) with \(\displaystyle \frac{b\sqrt{3}}{2}\).  This gives you:

\(\displaystyle 56.25\sqrt{3}=\frac{1}{2}*b*\frac{b\sqrt{3}}{2}=\frac{b^2\sqrt{3}}{4}\)

Now, let's write that a bit more simply:

\(\displaystyle 56.25\sqrt{3}=\frac{b^2\sqrt{3}}{4}\)

Solve for \(\displaystyle b\). Begin by multiplying each side by \(\displaystyle 4\):

\(\displaystyle 225\sqrt{3}=b^2\sqrt{3}\)

Divide each side by \(\displaystyle \sqrt3\):

\(\displaystyle b^2 = 225\)

Finally, take the square root of both sides. This gives you \(\displaystyle b=15\). Therefore, the perimeter is \(\displaystyle 15*3 = 45\).

Example Question #2 : Equilateral Triangles

An equilateral triangle with a perimeter of \(\displaystyle 48 cm\) has sides with what length?

Possible Answers:

\(\displaystyle 12cm\)

\(\displaystyle 8 cm\)

\(\displaystyle 16 cm\)

\(\displaystyle 24 cm\)

\(\displaystyle 48 cm\)

Correct answer:

\(\displaystyle 16 cm\)

Explanation:

An equilateral triangle has 3 equal length sides.

Therefore the perimeter equation is as follows,

\(\displaystyle P=3s\).

So divide the perimeter by 3 to find the length of each side.

Thus the answer is:

\(\displaystyle 48=3s\)

\(\displaystyle \frac{48}{3}=s\)

\(\displaystyle 16 cm\)

Example Question #2 : Equilateral Triangles

Jill has an equilateral triangular garden with a base of \(\displaystyle 5x-1\) and one leg with a length of \(\displaystyle 3x+13\), what is the perimeter? 

Possible Answers:

\(\displaystyle 34\)

\(\displaystyle 68\)

\(\displaystyle 578\)

\(\displaystyle 7\)

\(\displaystyle 102\)

Correct answer:

\(\displaystyle 102\)

Explanation:

Since the triangle is equilateral, the base and the legs are equal, so the first step is to set the two equations equal to each other. Start with \(\displaystyle 5x-1=3x+13\), add \(\displaystyle 1\) to both sides giving you \(\displaystyle 5x=3x+14\). Subtract \(\displaystyle 3x\) from both sides, leaving \(\displaystyle 2x=14\). Finally divide both sides by \(\displaystyle 2\), so you're left with \(\displaystyle x=7\). Plug \(\displaystyle 7\) back in for \(\displaystyle x\)  into either of the equations so that you get a side length of \(\displaystyle 34\). To find the perimeter, multiply the side length \(\displaystyle \left ( 34\right )\), by \(\displaystyle 3\), giving you \(\displaystyle 102\).

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