SAT Math : Plane Geometry

Study concepts, example questions & explanations for SAT Math

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Example Questions

Example Question #13 : Rectangles

The length and width of a rectangle are in the ratio of 3:4. If the rectangle has an area of 108 square centimeters, what is the length of the diagonal?

Possible Answers:

12 centimeters

24 centimeters

9 centimeters

15 centimeters

18 centimeters

Correct answer:

15 centimeters

Explanation:

The length and width of the rectangle are in a ratio of 3:4, so the sides can be written as 3x and 4x.

We also know the area, so we write an equation and solve for x:

(3x)(4x) = 12x= 108.

x2 = 9

x = 3

Now we can recalculate the length and the width:

length = 3x = 3(3) = 9 centimeters

width = 4x = 4(3) = 12 centimeters

Using the Pythagorean Theorem we can find the diagonal, c:

length2 + width2 = c2

92 + 12= c2 

81 + 144 = c2

225 = c2

= 15 centimeters

Example Question #21 : Rectangles

Find the length of the diagonal of a rectangle whose sides are 8 and 15.

Possible Answers:

\(\displaystyle \sqrt{17}\)

\(\displaystyle \sqrt{23}\)

\(\displaystyle 23\)

\(\displaystyle 17\)

Correct answer:

\(\displaystyle 17\)

Explanation:

To solve. simply use the Pythagorean Theorem where \(\displaystyle a=8\) and \(\displaystyle b=15\)

Thus,

\(\displaystyle c=\sqrt{a^2+b^2}=\sqrt{8^2+15^2}=\sqrt{289}=17\)

Example Question #202 : Geometry

Prism

The above figure depicts a cube, each edge of which has length 18. Give the length of the shortest path from Point A to Point B that lies completely along the surface of the cube.

Possible Answers:

\(\displaystyle 18\sqrt{5}\)

\(\displaystyle 18 \sqrt{2}\)

\(\displaystyle 18\sqrt{3}\)

\(\displaystyle 36\)

\(\displaystyle 18\sqrt{6}\)

Correct answer:

\(\displaystyle 18\sqrt{5}\)

Explanation:

The shortest path is along two of the surfaces of the prism. There are three possible choices - top and front, right and front, and rear and bottom - but as it turns out, since all faces are (congruent) squares, all three paths have the same length. One such path is shown below, with the relevant faces folded out:

 Prism 2 

The length of the path can be seen to be equal to that of the diagonal of a rectangle with length and width 18 and 36, so its length can be found by applying the Pythagorean Theorem. Substituting 18 and 36 for \(\displaystyle a\) and \(\displaystyle b\):

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

\(\displaystyle = \sqrt{18^{2}+36^{2}}\)

\(\displaystyle = \sqrt{324+1,296}\)

\(\displaystyle = \sqrt{1,620 }\)

Applying the Product of Radicals Rule:

\(\displaystyle c = \sqrt{1,620 } = \sqrt{324} \cdot \sqrt{5 } = 18 \sqrt{5 }\).

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

 

 

 

The two rectangles shown below are similar. What is the length of EF?

 Sat_mah_166_02

Possible Answers:

8

5

10

6

Correct answer:

10

Explanation:

When two polygons are similar, the lengths of their corresponding sides are proportional to each other.  In this diagram, AC and EG are corresponding sides and AB and EF are corresponding sides. 

To solve this question, you can therefore write a proportion:

AC/EG = AB/EF ≥ 3/6 = 5/EF

From this proportion, we know that side EF is equal to 10.

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

A rectangle is x inches long and 3x inches wide.  If the area of the rectangle is 108, what is the value of x?

Possible Answers:

4

8

3

12

6

Correct answer:

6

Explanation:

Solve for x

Area of a rectangle A = lw = x(3x) = 3x2 = 108

x2 = 36

x = 6

Example Question #31 : Quadrilaterals

If the area of rectangle is 52 meters squared and the perimeter of the same rectangle is 34 meters. What is the length of the larger side of the rectangle if the sides are integers?

Possible Answers:

16 meters

14 meters

12 meters

15 meters

13 meters

Correct answer:

13 meters

Explanation:

Area of a rectangle is = lw

Perimeter = 2(l+w)

We are given 34 = 2(l+w) or 17 = (l+w)

possible combinations of l + w

are 1+16, 2+15, 3+14, 4+13... ect

We are also given the area of the rectangle is 52 meters squared.

Do any of the above combinations when multiplied together= 52 meters squared? yes 4x13 = 52

Therefore the longest side of the rectangle is 13 meters

 

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

Rectangles a

Figure is not drawn to scale.

The provided figure is a rectangle divided into two smaller rectangles, with

Rectangle \(\displaystyle ABEF \sim\) Rectangle \(\displaystyle CDEB\).

Which expression is equal to the length of \(\displaystyle \overline{FE}\)?

Possible Answers:

\(\displaystyle 12+ 2 \sqrt{5}\)

\(\displaystyle 18\)

\(\displaystyle 16\)

\(\displaystyle 12 + 4\sqrt{5}\)

\(\displaystyle 12+\sqrt{5}\)

Correct answer:

\(\displaystyle 12 + 4\sqrt{5}\)

Explanation:

Since Rectangle \(\displaystyle ABEF\) is similar to Rectangle \(\displaystyle CDEB\), it follows that corresponding sides are in proportion. Specifically,

\(\displaystyle \frac{FE}{BE} = \frac{BE}{DE}\)

\(\displaystyle FE + DE = FD\);

since \(\displaystyle FD = 24\), if we let \(\displaystyle t = FE\), then 

\(\displaystyle t+ DE = 24\),

and 

\(\displaystyle DE= 24 - t\)

Setting \(\displaystyle BE = 8\)\(\displaystyle FE = t\), and \(\displaystyle DE= 24 - t\), the proportion statement becomes

\(\displaystyle \frac{t}{8} = \frac{8}{24-t }\)

Cross-multiplying, we get

\(\displaystyle t(24-t) = 8 \cdot 8\)

Simplifying, we get

\(\displaystyle 24t-t ^{2}= 64\)

Since this is quadratic, all terms must be moved to one side:

\(\displaystyle 24t-t ^{2} - 24t + t ^{2} = 64 - 24t + t ^{2}\)

\(\displaystyle 0= 64 - 24t + t ^{2}\)

or

\(\displaystyle t ^{2} - 24t + 64 = 0\)

The solutions to quadratic equation

\(\displaystyle ax^{2} + bx + c = 0\)

can be found by way of the quadratic formula

\(\displaystyle x = \frac{-b \pm \sqrt{b^{2}-4ac}}{2a}\)

Set \(\displaystyle a = 1, b= -24, c= 64\):

\(\displaystyle x = \frac{-(-24) \pm \sqrt{(-24)^{2}-4(1)(64)}}{2(1)}\)

\(\displaystyle = \frac{ 24 \pm \sqrt{576-256}}{2}\)

\(\displaystyle = \frac{ 24 \pm \sqrt{320}}{2}\)

Simplifying the radical using the Product of Radicals Property, we get

\(\displaystyle \frac{ 24 \pm \sqrt{64}\cdot \sqrt{5}}{2}\)

\(\displaystyle =\frac{ 24 \pm8\sqrt{5}}{2}\)

Splitting the fraction and reducing:

\(\displaystyle \frac{ 24 }{2} \pm \frac{ 8\sqrt{5}}{2}\)

\(\displaystyle =12 \pm 4\sqrt{5}\)

This actually tells us that the lengths of the two segments \(\displaystyle \overline{FE}\) and \(\displaystyle \overline{ED}\) have the lengths \(\displaystyle 12 + 4\sqrt{5}\) and \(\displaystyle 12 -4\sqrt{5}\)\(\displaystyle \overline{FE}\) is seen in the diagram to be the longer, so we choose the greater value, \(\displaystyle 12 + 4\sqrt{5}\).

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

A rectangle has a width of 2x. If the length is five more than 150% of the width, what is the perimeter of the rectangle?

Possible Answers:

6x2 + 5

6x2 + 10x

5x + 5

5x + 10

10(x + 1)

Correct answer:

10(x + 1)

Explanation:

Given that w = 2x and l = 1.5w + 5, a substitution will show that l = 1.5(2x) + 5 = 3x + 5.  

P = 2w + 2l = 2(2x) + 2(3x + 5) = 4x + 6x + 10 = 10x + 10 = 10(x + 1)

Example Question #201 : Geometry

Find the perimeter of a rectangle with width 7 and length 9.

Possible Answers:

\(\displaystyle 63\)

\(\displaystyle 32\)

\(\displaystyle \frac{63}{2}\)

\(\displaystyle 16\)

Correct answer:

\(\displaystyle 32\)

Explanation:

To solve, simply use the formula for the perimeter of a rectangle.

Substitute in the width of seven and the length of nine.

Thus,

\(\displaystyle P=2(w+l)=2*(7+9)=2*16=32\)

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

Find the perimeter of a rectangle whose side lengths are 1 and 2.

Possible Answers:

\(\displaystyle 12\)

\(\displaystyle 2\)

\(\displaystyle 4\)

\(\displaystyle 6\)

Correct answer:

\(\displaystyle 6\)

Explanation:

To solve, simply use the formula for the perimeter of a rectangle. Thus,

\(\displaystyle P=2(w+l)=2*(1+2)=2*3=6\)

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