GMAT Math : Calculating the area of an equilateral triangle

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Example Questions

Example Question #301 : Geometry

Three straight sticks are gathered of exactly equal length. They are placed end to end on the ground to form a triangle. If the area of the triangle they form is 1.732 square feet. What is the length in feet of each stick?

Possible Answers:

\(\displaystyle 1.5\ feet\)

\(\displaystyle 3\ feet\)

\(\displaystyle 2.5\ feet\)

\(\displaystyle 1\ foot\)

\(\displaystyle 2\ feet\)

Correct answer:

\(\displaystyle 2\ feet\)

Explanation:

Equilateral_triangle

Let \(\displaystyle s\) be the length of a side of an equilateral triangle. Then the formula for the area of an equilateral triangle with side \(\displaystyle s\) is

 \(\displaystyle \frac{s^2\sqrt{3}}{4}\)

So solving \(\displaystyle 1.732 = \frac{s^2\sqrt{3}}{4}\) 

we get \(\displaystyle s=2\).

 

Alternative Solution:

Without knowing this formula you can still use the Pythagorean Theorem to solve this. By drawing the height of the triangle, you split the triangle into 2 right triangles of equal size. The sides are the height, \(\displaystyle \frac{s}{2}\) and \(\displaystyle s\). Letting \(\displaystyle h\) stand for the unknown height, we solve 

\(\displaystyle (\frac{s}{2})^2 + h^2= s^2\) solving for \(\displaystyle h\) we get

 \(\displaystyle h=\sqrt(s^2-(\frac{s}{2})^2) = \sqrt(s^2-\frac{s^2}{4}) = \sqrt(\frac{3s^2}{4}) = \frac{s\sqrt(3)}{2}\)

The area for any triangle is the base times the height divided by 2. So

 \(\displaystyle 1.732=\frac{sh}{2} = \frac{s(s\sqrt(3))}{2*2} = \frac{s^2\sqrt(3)}{4}\) or \(\displaystyle s=2\).

Example Question #302 : Geometry

If an equilateral triangle has a side length of \(\displaystyle 3\) and a height of \(\displaystyle 2\), what is the area of the given triangle?

Possible Answers:

\(\displaystyle 9\)

\(\displaystyle 6\)

\(\displaystyle 3\)

\(\displaystyle 12\)

Correct answer:

\(\displaystyle 3\)

Explanation:

To find the area of a traingle, we need the height and base lengths. Plug the given values into the following formula:

\(\displaystyle A= \frac{bh}{2}\)

\(\displaystyle = \frac{(3*2)}2 {}\)

\(\displaystyle =3\)

Example Question #1 : Equilateral Triangles

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Triangle \(\displaystyle ABC\) is an equilateral triangle with side length \(\displaystyle 2\). What is the area of the triangle?

Possible Answers:

\(\displaystyle \sqrt{3}\)

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

\(\displaystyle 3\)

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

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

Correct answer:

\(\displaystyle \sqrt{3}\)

Explanation:

The area of an equilateral triangle is given by the following formula:

 \(\displaystyle \frac{s^{2}\sqrt{3}}{4}\), where \(\displaystyle s\) is the length of a side.

Since we know the length of the side, we can simply plug it in the formula and we have \(\displaystyle \frac{4\sqrt{3}}{4}\) or \(\displaystyle \sqrt{3}\), which is the final answer.

Example Question #1 : Calculating The Area Of An Equilateral Triangle

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\(\displaystyle ABC\) is an equilateral triangle inscribed in a cirlce with radius \(\displaystyle 3\). What is the area of the triangle \(\displaystyle ABC\)?

Possible Answers:

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

\(\displaystyle 27\)

\(\displaystyle 27\frac{\sqrt{3}}{4}\)

\(\displaystyle 27\frac{\sqrt{5}}{4}\)

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

Correct answer:

\(\displaystyle 27\frac{\sqrt{3}}{4}\)

Explanation:

Since we are given a radius for the circle, we should be able to find the length of the height of the equilateral triangle, indeed, the center of the circle is \(\displaystyle \frac{2}{3}\) of the length of the height from any vertex.

Therefore, the height is \(\displaystyle 3=\frac{2}{3}\cdot h\) where \(\displaystyle h\) is the length of the height of the triangle. Therefore \(\displaystyle h=\frac{9}{2}\).

We can now plug in this value in the formula of the height of an equilateral triangle\(\displaystyle h=s\frac{\sqrt{3}}{2}\), where \(\displaystyle s\) is the length of the side of the triangle.

Therefore, \(\displaystyle s=\frac{9}{\sqrt{3}}\) or \(\displaystyle 3\sqrt{3}\).

Now we should plug in this value into the formula for the area of an equilateral triangle \(\displaystyle a=s^{2}\frac{\sqrt{3}}{4}\) where \(\displaystyle a\) is the value of the area of the equilateral triangle. Therefore \(\displaystyle a= 27\frac{\sqrt{3}}{4}\), which is our final answer. 

Example Question #2 : Calculating The Area Of An Equilateral Triangle

A given equilateral triangle has a side length \(\displaystyle 4\) and a height \(\displaystyle 7\) . What is the area of the triangle?

Possible Answers:

\(\displaystyle 11\)

Not enough information provided

\(\displaystyle 14\)

\(\displaystyle 22\)

\(\displaystyle 28\)

Correct answer:

\(\displaystyle 14\)

Explanation:

For a given equilateral triangle with a side length \(\displaystyle b\) and a height \(\displaystyle h\), the area \(\displaystyle A\) is 

\(\displaystyle A=\frac{1}{2}bh\). Plugging in the values provided:

\(\displaystyle A=\frac{1}{2}(4)(7)\)

\(\displaystyle A=\frac{1}{2}(28)\)

\(\displaystyle A=14\)

 

Example Question #2 : Calculating The Area Of An Equilateral Triangle

A given right triangle has a base length \(\displaystyle 5\) and a height \(\displaystyle 4\) . What is the area of the triangle?

Possible Answers:

Not enough information to solve

\(\displaystyle 20\)

\(\displaystyle 10\)

\(\displaystyle 18\)

\(\displaystyle 9\)

Correct answer:

\(\displaystyle 10\)

Explanation:

For a given right triangle with a side length \(\displaystyle b\) and a height \(\displaystyle h\), the area \(\displaystyle A\) is 

\(\displaystyle A=\frac{1}{2}bh\). Plugging in the values provided:

\(\displaystyle A=\frac{1}{2}(5)(4)\)

\(\displaystyle A=\frac{1}{2}(20)\)

\(\displaystyle A=10\)

Example Question #1 : Calculating The Area Of An Equilateral Triangle

A given right triangle has a base of length \(\displaystyle 9\) and a height \(\displaystyle 8\) . What is the area of the triangle?

Possible Answers:

\(\displaystyle 36\)

\(\displaystyle 17\)

\(\displaystyle 34\)

Not enough information to solve

\(\displaystyle 72\)

Correct answer:

\(\displaystyle 36\)

Explanation:

For a given right triangle with a side length \(\displaystyle b\) and a height \(\displaystyle h\), the area \(\displaystyle A\) is 

\(\displaystyle A=\frac{1}{2}bh\). Plugging in the values provided:

\(\displaystyle A=\frac{1}{2}(9)(8)\)

\(\displaystyle A=\frac{1}{2}(72)\)

\(\displaystyle A=36\)

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