SSAT Upper Level Math : Areas and Perimeters of Polygons

Study concepts, example questions & explanations for SSAT Upper Level Math

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Example Questions

Example Question #10 : How To Find The Area Of A Square

The perimeter of a square is equal to the circumference of a circle with area 4. What is the area of the square?

Possible Answers:

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

\(\displaystyle \pi\)

\(\displaystyle 4 \pi\)

\(\displaystyle \sqrt{\pi}\)

Correct answer:

\(\displaystyle \pi\)

Explanation:

First, we need the radius \(\displaystyle r\) of the circle, which can be determined from the area of a circle formula by setting \(\displaystyle A = 4\):

\(\displaystyle \pi r ^{2} = A\)

\(\displaystyle \pi r ^{2} = 4\)

\(\displaystyle \frac{ \pi r ^{2}}{\pi} = \frac{4 }{\pi}\)

\(\displaystyle r ^{2}= \frac{4 }{\pi}\)

\(\displaystyle r = \sqrt{\frac{4 }{\pi} }\)

Simplifying the expression by splitting the radicand and rationalizing the denominator:

\(\displaystyle r = \frac{\sqrt{4}}{\sqrt{\pi}}\)

\(\displaystyle r = \frac{2}{\sqrt{\pi}}\)

\(\displaystyle r = \frac{2 \cdot \sqrt{\pi}}{\sqrt{\pi} \cdot \sqrt{\pi}}\)

\(\displaystyle r= \frac{2 \sqrt{\pi}}{\pi}\)

The circumference of the circle is \(\displaystyle 2 \pi\) multiplied by the radius, or

\(\displaystyle C = 2 \pi r\)

\(\displaystyle C = 2 \pi \cdot \frac{2 \sqrt{\pi}}{\pi} = 4 \sqrt{\pi}\)

This is also the perimeter of the square, so the length of each side is one fourth of this perimeter, or

\(\displaystyle \frac{ 4 \sqrt{\pi}}{4} = \sqrt{\pi}\)

The area of the square is the square of this common sidelength, or 

\(\displaystyle \left ( \sqrt{\pi} \right )^{2} = \pi\).

Example Question #32 : Area Of Polygons

A square and a circle have the same area. The circle has diameter 10. Which expression is equal to the length of one side of the square?

Possible Answers:

\(\displaystyle \pi \sqrt{5}\)

\(\displaystyle 5\sqrt{ \pi}\)

\(\displaystyle 10 \sqrt{ \pi}\)

\(\displaystyle \pi \sqrt{10}\)

Correct answer:

\(\displaystyle 5\sqrt{ \pi}\)

Explanation:

A circle with diameter 10 has radius half this, or 5. The area of the circle can be found using the formula

\(\displaystyle A = \pi r ^{2}\),

setting \(\displaystyle r = 5\):

\(\displaystyle A = \pi \cdot 5^{2} = 25 \pi\).

The square also has this area. The length of one side of this square is equal to the square root of this, which is

\(\displaystyle s = \sqrt{A} = \sqrt{ 25 \pi}\)

Simplify this by breaking the radicand, as follows:

\(\displaystyle S = \sqrt{ 25 \pi} = \sqrt{ 25} \cdot \sqrt{ \pi} = 5\sqrt{ \pi}\)

Example Question #61 : Areas And Perimeters Of Polygons

Right triangle 6

A square has the same area as the above right triangle. Give the length of one side of the square.

Possible Answers:

\(\displaystyle \sqrt{65}\)

\(\displaystyle \sqrt{30}\)

\(\displaystyle 2 \sqrt{15}\)

\(\displaystyle 2 \sqrt{13}\)

Correct answer:

\(\displaystyle \sqrt{30}\)

Explanation:

The area of a right triangle is equal to half the product of the lengths of its legs. Since we know the lengths of one leg and the hypotenuse, we can calculate the length of the other leg using the Pythagorean Theorem. We can use this form:

\(\displaystyle b = \sqrt{ c^{2} - a ^{2}}\)

Setting \(\displaystyle c\) and \(\displaystyle a\) equal to the lengths of the hypotenuse and the known leg - 13 and 5, respectively:

\(\displaystyle b = \sqrt{ 13^{2} - 5 ^{2}} = \sqrt{169 - 25} = \sqrt{144} = 12\)

The area of the above triangle is

\(\displaystyle A = \frac{1}{2} ab = \frac{1}{2} \cdot 5 \cdot 12 = 30\)

The square also has this area. The length of one side of this square is equal to the square root of this, which is \(\displaystyle \sqrt{30 }\).

Example Question #1 : How To Find The Perimeter Of A Rectangle

The length and width of a rectangle are \(\displaystyle x + 16\) and \(\displaystyle 2x-7\). Give its perimeter in terms of \(\displaystyle x\).

Possible Answers:

\(\displaystyle 2x^{2} +25x -112\)

\(\displaystyle 6x+18\)

\(\displaystyle 3x+9\)

\(\displaystyle 2x^{2} -25x -112\)

\(\displaystyle 6x+46\)

Correct answer:

\(\displaystyle 6x+18\)

Explanation:

A rectangle has perimeter \(\displaystyle P = 2L + 2W\)\(\displaystyle L\) the length and \(\displaystyle W\) the width. Substitute \(\displaystyle L= x + 16\) and \(\displaystyle W = 2x-7\) in the perimeter formula, and simplify.

\(\displaystyle P = 2L + 2W\)

\(\displaystyle = 2 \left ( x+16\right ) + 2 \left ( 2x-7\right )\)

\(\displaystyle = 2 \cdot x+2 \cdot 16 +2 \cdot 2x-2 \cdot 7\)

\(\displaystyle = 2x+32 +4x-14\)

\(\displaystyle = 6x+18\)

Example Question #2 : How To Find The Perimeter Of A Rectangle

A rectangle has length 30 inches and width 25 inches. Which of the following is true about its perimeter?

Possible Answers:

Its perimeter is between 5 and 6 feet.

Its perimeter is between 8 and 9 feet.

Its perimeter is between 10 and 11 feet.

Its perimeter is between 9 and 10 feet.

Its perimeter is between 4 and 5 feet.

Correct answer:

Its perimeter is between 9 and 10 feet.

Explanation:

In inches, the perimeter of the rectangle can be calculated by substituting \(\displaystyle L = 30, W = 25\) in the following formula:

\(\displaystyle P = 2 (L + W)\)

\(\displaystyle P = 2 (30 + 25) = 2\times 55 = 110\)

The perimeter is 110 inches.

Now divide by 12 to convert to feet:

\(\displaystyle 110 \div 12 = 9 \textrm{ R }2\)

This makes the perimeter 9 feet 2 inches, which is between 9 feet and 10 feet.

Example Question #2 : How To Find The Perimeter Of A Rectangle

Perimeter of a rectangle is 36 inches. If the width of the rectangle is 3 inches less than its length, give the length and width of the rectangle.

Possible Answers:

\(\displaystyle L=10, W=7\) inches

\(\displaystyle L=10.5, W=7.5\) inches

\(\displaystyle L=12, W=9\) inches

\(\displaystyle L=11, W=8\) inches

\(\displaystyle L=11.5, W=8.5\) inches

Correct answer:

\(\displaystyle L=10.5, W=7.5\) inches

Explanation:

Let:

\(\displaystyle Length=x\Rightarrow Width=x-3\)

The perimeter of a rectangle is \(\displaystyle P=2L+2W\), where \(\displaystyle L\) is the length and \(\displaystyle W\) is the width of the rectangle. The perimeter is known so we can set up an equation in terms of \(\displaystyle x\) and solve it:

 

\(\displaystyle P=2L+2W=2(x)+2(x-3)=36\)

\(\displaystyle \Rightarrow2x+2x-6=36\)

\(\displaystyle \Rightarrow 4x-6=36\)

\(\displaystyle \Rightarrow 4x=42\)

\(\displaystyle \Rightarrow x=10.5\)

So we can get:

\(\displaystyle Length=x=10.5\) inches

\(\displaystyle Width=x-3=10.5-3=7.5\) inches

 

 

Example Question #801 : Geometry

The length and width of a rectangle are \(\displaystyle t+1\) and \(\displaystyle t-1\), respectively. Give its perimeter in terms of \(\displaystyle t\).

Possible Answers:

\(\displaystyle 2t+2\)

\(\displaystyle 4t+4\)

\(\displaystyle 2t-2\)

\(\displaystyle 4t\)

\(\displaystyle 4t-4\)

Correct answer:

\(\displaystyle 4t\)

Explanation:

The perimeter of a rectangle is \(\displaystyle P=2L+2W\), where \(\displaystyle L\) is the length and \(\displaystyle W\) is the width of the rectangle. In order to find the perimeter we can substitute the \(\displaystyle L=t+1\) and \(\displaystyle W=t-1\) in the perimeter formula:

\(\displaystyle P=2L+2W\)

\(\displaystyle =2(t+1)+2(t-1)\)

\(\displaystyle =2t+2+2t-2\)

\(\displaystyle =4t\)

Example Question #3 : How To Find The Perimeter Of A Rectangle

The length of a rectangle is \(\displaystyle 2x+4\) and the width of this rectangle is \(\displaystyle 3\) meters shorter than its length. Give its perimeter in terms of \(\displaystyle x\).

Possible Answers:

\(\displaystyle 10x+6\)

\(\displaystyle 8x+8\)

\(\displaystyle 10x+8\)

\(\displaystyle 8x+6\)

\(\displaystyle 8x+10\)

Correct answer:

\(\displaystyle 8x+10\)

Explanation:

The length of the rectangle is known, so we can find the width in terms of \(\displaystyle x\):

 

 \(\displaystyle Width=Length-3=(2x+4)-3=2x+1\)

 

The perimeter of a rectangle is \(\displaystyle P=2L+2W\), where \(\displaystyle L\) is the length and \(\displaystyle W\) is the width of the rectangle.

 

In order to find the perimeter we can substitute the \(\displaystyle L=2x+4\) and \(\displaystyle W=2x+1\) in the perimeter formula:

 

\(\displaystyle P=2L+2W\)

\(\displaystyle =2(2x+4)+2(2x+1)\)

\(\displaystyle =4x+8+4x+2\)

\(\displaystyle =8x+10\)

Example Question #803 : Geometry

A rectangle has a length of \(\displaystyle t^2\) inches and a width of \(\displaystyle 4t\) inches. Which of the following is true about the rectangle perimeter if \(\displaystyle t=5\) ?

Possible Answers:

Its perimeter is between 8 and 9 feet.

Its perimeter is between 7 and 8 feet.

Its perimeter is more than 8 feet.

Its perimeter is between 7.2 and 7.4 feet.

Its perimeter is less than 7 feet.

Correct answer:

Its perimeter is between 7 and 8 feet.

Explanation:

Substitute \(\displaystyle t=5\) to get \(\displaystyle L\) and \(\displaystyle W\):

 

\(\displaystyle L=t^2=5^2=25\)

\(\displaystyle W=5t=5\times 4=20\)

 

The perimeter of a rectangle is \(\displaystyle P=2L+2W\) , where \(\displaystyle L\) is the length and \(\displaystyle W\) is the width of the rectangle. So we have:

 

\(\displaystyle P=2L+2W=2\times 25+2\times20=50+40=90\) inches

 

Now we should divide the perimeter by 12 in order to convert to feet:

 

\(\displaystyle Perimeter=90\div 12=7.5\) feet

 

So the perimeter is 7 feet and 6 inches, which is between 7 and 8 feet.

Example Question #3 : How To Find The Perimeter Of A Rectangle

Which of these polygons has the same perimeter as a rectangle with length 55 inches and width 15 inches?

Possible Answers:

The other answer choices are incorrect.

A regular heptagon with sidelength two feet

A regular octagon with sidelength two feet

A regular hexagon with sidelength two feet

A regular pentagon with sidelength two feet

Correct answer:

The other answer choices are incorrect.

Explanation:

The perimeter of a rectangle is twice the sum of its length and its width; a rectangle with dimensions 55 inches and 15 inches has perimeter 

\(\displaystyle 2 (55+15) = 140\) inches.

All of the polygons in the choices are regular - that is, all have congruent sides - and all have sidelength two feet, or 24 inches, so we divide 140 by 24 to determine how many sides such a polygon would need to have a perimeter equal to the rectangle. However, 

\(\displaystyle 140 \div 24 = 5 \frac{5}{6}\),

so there cannot be a regular polygon with these characteristics. All of the choices fail, so the correct response is that none are correct.

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