Area - SAT Subject Test in Math II

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Question

Note: Figure NOT drawn to scale.

Refer to the above diagram. , , and $\angle ABC$ and $\angle ADB$ are right angles. What percent of $\Delta ABC$ is colored red?

Answer

, as the length of the altitude corresponding to the hypotenuse, is the geometric mean of the lengths of the parts of the hypotenuse it forms; that is, it is the square root of the product of the two:

$BD =\sqrt{ \left ( AD\right ) \left ( DC\right )} = \sqrt{6 \cdot 24} = \sqrt{144} = 12$ .

The area of $\Delta ADB$ , the shaded region, is half the products of its legs:

$A= \frac{1}{2} (AD) \times (DB ) =\frac{1}{2} \times 6 \times 12 = 36$

The area of $\Delta ABC$ is half the product of its hypoteuse, which we can see as the base, and the length of corresponding altitude $\overline{BD}$ :

$A = \frac{1}{2} (AC) (BD) = \frac{1}{2} \cdot 30 \cdot 12 = 180$

$\Delta ADB$ comprises

$\frac{36}{180} \times \100 = 20 %$

of $\Delta ADC$ .

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Question

Note: Figure NOT drawn to scale

Refer to the above figure, which shows a square garden (in green) surrounded by a dirt path (in orange). The dirt path is seven feet wide throughout. What is the area of the dirt path in square feet?

Answer

The area of the dirt path is the area of the outer square minus that of the inner square.

The outer square has sidelength 75 feet and therefore has area

$75 \times 75 = 5,625$ square feet.

The inner square has sidelength $75 - 2 \times 7 = 61$ feet and therefore has area

$61 \times 61 = 3,721$ square feet.

Subtract to get the area of the dirt path:

square feet.

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Question

Refer to the above figure, which shows a rectangular garden (in green) surrounded by a dirt path (in orange). The dirt path is six feet wide throughout. Which of the following polynomials gives the area of the garden in square feet?

Answer

The length of the garden is $2 \times 6= 12$ feet less than that of the entire lot, or

;

The width of the garden is $2 \times 6= 12$ less than that of the entire lot, or

;

The area of the garden is their product:

$=2x \cdot x - 2x \cdot 12 - 12 \cdot x + 12 \cdot 12$

$= 2x ^{2} - 24x - 12 x + 144$

$= 2x ^{2} - 36 x + 144$

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Question

The circle in the above diagram has its center at the origin. To the nearest tenth, what is the area of the pink region?

Answer

First, it is necessary to determine the radius of the circle. This is the distance between and , so we apply the distance formula:

$r = \sqrt{(x_{2} - x _{1})^{2}+(y_{2} - y_{1})^{2}}$

$r = \sqrt{\left ( -8 -0 \right )^{2} +\left ( 6-0\right )^{2} }=\sqrt{ 8^{2}+6^{2}} = \sqrt{64+36 }= \sqrt{100} = 10$

Subsequently, the area of the circle is

$A = \pi r^{2} = \pi \cdot 10^{2} = 100 \pi$

Now, we need to find the central angle of the shaded sector. This is found using the relationship

$\tan \theta = \frac{y}{x}$

$\tan \theta = \frac{6}{-8} = -0.75$

Using a calculator, we find that $\tan^{-1}(-0.75) \approx -36.87^{\circ }$ ; since we want a degree measure between $90^{\circ }$ and $180 ^{\circ }$ , we adjust by adding $180 ^{\circ }$ , so

$\theta \approx \left (-36.87+ 180 \right ) ^{\circ } \approx 143.13^{\circ }$

The area of the sector is calculated as follows:

$A \approx \frac{143.13 }{360} \cdot 100 \pi \approx 124.9$

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Question

The above figure is a regular decagon. If , then to the nearest whole number, what is ?

Answer

As an interior angle of a regular decagon, $\angle B$ measures

$m \angle B = \left [\frac{180(10-2)}{10} \right ] ^{\circ }= 144^{\circ }$ .

.

can be found using the Law of Cosines:

$\left ( AC\right )^{2} = \left ( AB\right )^{2}+ \left ( BC\right )^{2} - 2 \left ( AB\right ) \left ( BC\right ) \cos m\angle B$

$\left ( AC\right )^{2} = 100^{2}+ 100^{2} - 2 \cdot 100 \cdot 100 \cos 144^{\circ }$

$\left ( AC\right )^{2} \approx 10,000 + 10,000 - 20,000 (-0.8090)$

$\left ( AC\right )^{2} \approx 20,000 - (-16,180) \approx 36,180$

$AC \approx \sqrt{36,180} \approx 190$

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Question

You own a mug with a circular bottom. If the distance around the outside of the mug's base is $20 \pi cm$ what is the area of the base?

Answer

You own a mug with a circular bottom. If the distance around the outside of the mug's base is $20 \pi cm$ what is the area of the base?

Begin by solving for the radius:

$C=2\pi r$

$20 \pi cm=2\pi r$

Next, plug the radius back into the area formula and solve:

$A_{circle}=\pi r^2$

$A_{circle}=\pi (10cm)^2=100 \pi cm^2$

So our answer is:

$100 \pi cm^2$

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Question

You have a right triangle with a hypotenuse of 13 inches and a leg of 5 inches, what is the area of the triangle?

Answer

You have a right triangle with a hypotenuse of 13 inches and a leg of 5 inches, what is the area of the triangle?

So find the area of a triangle, we need the following formula:

$A_{tri}=\frac{1}{2}bh$

However, we only know one leg, so we only know b or h.

To find the other leg, we can either use Pythagorean Theorem, or recognize that this is a 5-12-13 triangle. Meaning, our final leg is 12 inches long.

To prove this:

$x=\sqrt{144in^2}=12in$

Now, we know both legs, let's just plug in and solve for area:

$A_{tri}=\frac{1}{2}(5in)(12in)=6in*5in=30in^2$

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Question

You have a rectangular-shaped rug which you want to put in your living room. If the rug is 12.5 feet long and 18 inches wide, what is the area of the rug?

Answer

You have a rectangular-shaped rug which you want to put in your living room. If the rug is 12.5 feet long and 18 inches wide, what is the area of the rug?

To begin, we need to realize two things.

Our given measurements are not in equivalent units, so we need to convert one of them before doing any solving.

The area of a rectangle is given by:

Now, let's convert 18 inches to feet, because it seems easier than 12.5 feet to inches:

$18inches*\frac{1ft}{12in}=1.5ft$

Now, using what we know from 2) we can find our answer

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Question

Give the area of $\bigtriangleup ABC$ to the nearest whole square unit, where:

$m \angle A = 124^{\circ }$

Answer

The area of a triangle with two sides of lengths and and included angle of measure $\gamma$ can be calculated using the formula

$A = \frac{1}{2} ab \sin \gamma$ .

Setting $a = AB = 34, b= AC = 27 , \gamma = m \angle A = 124^{\circ }$ and evaluating :

$A = \frac{1}{2} (34 )(27)\sin 124 ^{\circ }$

$\approx 459 ( 0.8290 )$

$\approx 381$

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Question

To the nearest whole, give the area of a regular pentagon with a perimeter of fifty.

Answer

In a regular pentagon, called Pentagon , construct the five perpendicular segments from each vertex to its opposite side, as shown below:

The segments divide the pentagon into ten congruent triangles.

In particular, examine $\bigtriangleup OXD$ . $\overline{OD}$ , a radius of the pentagon, bisects $\angle EDC$ , which, as the interior angle of a regular pentagon, has measure $108 ^{\circ }$ ; therefore, $m \angle ODX = 54 ^{\circ }$ . $\overline{OX}$ is an apothem and therefore bisects $\overline{CD}$ ; since the pentagon has perimeter 50, $\overline{CD}$ has length one fifth of this, or 10, and .

Using trigonometry,

$\tan \angle ODX = \frac{OX}{XD }$ ,

or, substituting,

$\frac{OX}{5} = \tan 54 ^{\circ }$

Solving for :

$5 \cdot \frac{OX}{5} =5 \cdot \tan 54 ^{\circ }$

$OX \approx 5 \cdot (1.3764 )$

$OX \approx 6.8819$

The area of this triangle is half the product of the lengths of legs $\overline{XD}$ and $\overline{OX}$ :

$A \approx \frac{1}{2} \cdot 5 \cdot 6.8819 \approx 17.2048$

Since the pentagon comprises ten triangles of this area, multiply:

$17.2048 \cdot 10 = 172 .048$

To the nearest whole, this is 172.

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Question

Give the area of $\bigtriangleup ABC$ to the nearest whole square unit, where:

Answer

The area of a triangle, given its three sidelengths, can be calculated using Heron's formula:

$A = \sqrt{s(s-a)(s-b)(s-c)}$ ,

where , , and are the lengths of the sides, and $s = \frac{a+b+ c}{2}$ .

Setting , , and , evaluate :

$s = \frac{20+26+32}{2} = \frac{78}{2} = 39$

and, substituting in Heron's formula:

$A = \sqrt{39(39-20)(39-26)(39-32)}$

$= \sqrt{39(19)(13)(7)}$

$= \sqrt{67,431 }$

$\approx 259.7$

To the nearest whole, this is 260.

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Question

In the provided diagram, hexagon is regular; and are the midpoints of their respective sides. The perimeter of the hexagon is ; what is the area of Quadrilateral ?

Answer

Quadrilateral is a trapezoid, so we need to find the lengths of its bases and its height.

The perimeter of the hexagon is , so each side of the hexagon measures one sixth of this, or .

Construct the diameters of the hexagon, which meet at center ; construct the apothem from to $\overline{BC}$ , with point of intersection . The diagram is below:

The six triangles formed by the diameters are equilateral, so , and . Quadrilateral is a trapezoid with bases of length 10 and 20. Since $\overline{MN}$ has its endpoints at the midpoints of the legs of Trapezoid , it follows that $\overline{MN}$ is a midsegment, and has as its length $MN = \frac{1}{2} (AD + BC) = \frac{1}{2} (20+ 10 ) = 15$ .

The trapezoid has bases of length and ; we now need to find its height. This is the measure of $\overline{PQ}$ , which is half the length of apothem $\overline{OP}$ . is the height of an equilateral triangle $\bigtriangleup BCO$ and, consequently, the long leg of a right triangle $\bigtriangleup BCO$ . By the 30-60-90 Theorem,

$OP = BP \cdot \sqrt{3} = \frac{1}{2} \cdot BC \cdot \sqrt{3} = \frac{1}{2} \cdot 10 \cdot \sqrt{3}=5\sqrt{3}$ .

$PQ = \frac{1}{2} \cdot OP = \frac{1}{2} \cdot 5 \sqrt{3} = \frac{5}{2} \sqrt{3}$

The area of a trapezoid of height and base lengths and is

$A = \frac{1}{2} (B+b) h$ ;

Setting $h =PQ = \frac{5}{2} \sqrt{3} , B = MN = 15, b = BC = 10$ :

$A = \frac{1}{2} (B+b) h$

$= \frac{1}{2} \cdot (15 + 10 ) \cdot \frac{5}{2} \sqrt{3}$

$= \frac{25}{2} \cdot \frac{5}{2} \sqrt{3}$

$= \frac{125}{4} \sqrt{3}$

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Question

Find the area of a triangle with a base length of and a height of $\frac{1}{2}x$ .

Answer

Write the formula for the area of a triangle.

$A=\frac{BH}{2}$

Substitute the dimensions.

$A = \frac{x^2\cdot \frac{1}{2}x}{2} = \frac{1}{2}(x^2\cdot \frac{1}{2}x)$

The answer is: $\frac{1}{4}x^3$

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Question

Find the area of a circle with a radius of $\frac{3}{2}$ .

Answer

Write the formula for the area of a circle.

$A=\pi r^2$

Substitute the radius into the equation.

$A=\pi (\frac{3}{2})^2 = \frac{9}{4} \pi$

The area is: $\frac{9}{4}\pi$

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Question

Determine the area of a rectangle if the length is and the height is $\frac{1}{4}(x+3)$ .

Answer

The area of a rectangle is:

Substitute the length and height into the formula.

$A = (x+3)\cdot \frac{1}{4} (x+3)$

We will move the constant to the front and apply the FOIL method to simplify the binomials.

$A = \frac{1}{4} (x+3)(x+3) = \frac{1}{4} (x^2+6x+9)$

Distribute the fraction through all the terms of the trinomial.

The answer is: $\frac{1}{4}x^2 + \frac{3}{2}x + \frac{9}{4}$

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Question

What's the area of triangle with a side of $\frac{1}{2}x+\frac{1}{4}$ and a height of ?

Answer

Write the formula for the area of a triangle.

$A= \frac{BH}{2}$

Substitute the dimensions.

$A= \frac{(\frac{1}{2}x+\frac{1}{4})(2x+2)}{2}=\frac{1}{2}(\frac{1}{2}x+\frac{1}{4})(2x+2)$

$A= (\frac{1}{2}x+\frac{1}{4})(x+1)$

Use the FOIL method to expand this.

$A = (\frac{1}{2}x)(x)+ (\frac{1}{2}x)(1)+ (\frac{1}{4})(x)+ (\frac{1}{4})(1)$

Simplify the terms.

$A = \frac{1}{2}x^2+\frac{1}{2}x+\frac{1}{4}x+\frac{1}{4}$

Combine like-terms.

The answer is: $\frac{1}{2}x^2+\frac{3}{4}x+\frac{1}{4}$

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Question

Determine the side of a square with an area of .

Answer

Write the formula for the area of a square.

Substitute the area into the equation.

Square root both sides.

$\sqrt{100x}= \sqrt{s^2}$

$s = 10\sqrt{x}$

The answer is: $10\sqrt{x}$

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Question

Find the area of a circle with a radius of .

Answer

The area of a circle is $A=\pi r^2$ .

Substitute the radius and solve for the area.

$A=\pi (3x^3)^2 = \pi (3x^3)(3x^3) = 9\pi x^6$

The answer is: $9\pi x^6$

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Question

Determine the area of a triangle with a base of 6, and a height of $\frac{2}{3}$ .

Answer

Write the formula for the area of a triangle.

$A =\frac{1}{2}bh$

Substitute the base and height into the equation.

$A =\frac{1}{2}(6)(\frac{2}{3}) =2$

The answer is:

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Question

Find the area of a circle with a diameter of .

Answer

Divide the diameter by two. This will be the radius.

$r=\frac{x}{2}$

Write the formula for the area of a circle.

$A=\pi r^2$

$A=\pi (\frac{x}{2})^2 = \pi (\frac{x^2}{4})$

The answer is: $\frac{\pi x^2}{4}$

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