PSAT Math : Trapezoids

Study concepts, example questions & explanations for PSAT Math

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Example Questions

Example Question #1 : How To Find The Area Of A Trapezoid

A trapezoid has a base of length 4, another base of length s, and a height of length s. A square has sides of length s. What is the value of s such that the area of the trapezoid and the area of the square are equal?

Possible Answers:

\displaystyle 2\sqrt{2}

\displaystyle \sqrt{2}

\displaystyle 4

\displaystyle 2

\displaystyle 1

Correct answer:

\displaystyle 4

Explanation:

In general, the formula for the area of a trapezoid is (1/2)(a + b)(h), where a and b are the lengths of the bases, and h is the length of the height. Thus, we can write the area for the trapezoid given in the problem as follows:

area of trapezoid = (1/2)(4 + s)(s)

Similarly, the area of a square with sides of length a is given by a2. Thus, the area of the square given in the problem is s2.

We now can set the area of the trapezoid equal to the area of the square and solve for s.

(1/2)(4 + s)(s) = s2

Multiply both sides by 2 to eliminate the 1/2.

(4 + s)(s) = 2s2

Distribute the s on the left.

4s + s2 = 2s2

Subtract s2 from both sides.

4s = s2

Because s must be a positive number, we can divide both sides by s.

4 = s

This means the value of s must be 4.

The answer is 4.

Example Question #2 : Quadrilaterals

Rectangle_3

Note: Figure NOT drawn to scale.

The white region in the above diagram is a trapezoid. What percent of the above rectangle, rounded to the nearest whole percent, is blue?

Possible Answers:

\displaystyle 85 \%

\displaystyle 65 \%

\displaystyle 80 \%

\displaystyle 70 \%

\displaystyle 75 \%

Correct answer:

\displaystyle 70 \%

Explanation:

The area of the entire rectangle is the product of its length and width, or

\displaystyle 100 \times 50 = 5,000.

The area of the white trapezoid is one half the product of its height and the sum of its base lengths, or 

\displaystyle \frac{1}{2} \times (64+100) \times 18 = 1,476

Therefore, the blue polygon has area 

\displaystyle 5,000 - 1,476= 3,524

This is 

\displaystyle \frac{3,524}{5,000} \times 100 = 70.48 \% of the rectangle.

Rounded, this is 70%.

Example Question #3 : Quadrilaterals

Thingy_3

Refer to the above diagram. \displaystyle BD = 6, DX = 9, CX = 24.

Give the area of Quadrilateral \displaystyle ABDC.

Possible Answers:

\displaystyle 108

\displaystyle 264

\displaystyle 90

\displaystyle 165

\displaystyle 240

Correct answer:

\displaystyle 165

Explanation:

 \displaystyle \angle C \cong \angle BDX, since both are right; by the Corresponding Angles Theorem,  \displaystyle \overline{AC} || \overline{BD}, and Quadrilateral \displaystyle ABDC is a trapezoid.

 

By the Angle-Angle Similarity Postulate, since 

\displaystyle \angle C \cong \angle BDX 

and

\displaystyle \angle X \cong \angle X (by reflexivity),

\displaystyle \Delta ACX \sim \Delta BDX

and since corresponding sides of similar triangles are in proportion,

\displaystyle \frac{AC}{CX} = \frac{BD}{DX}

\displaystyle \frac{AC}{24} = \frac{6}{9}

\displaystyle \frac{AC}{24} \times 24 = \frac{6}{9} \times 24

\displaystyle AC = 16, the larger base of the trapozoid;

The smaller base is \displaystyle BD = 6.

\displaystyle CD = CX - DX = 24-9 = 15, the height of the trapezoid.

The area of the trapezoid is 

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

\displaystyle A = \frac{1}{2} (BD + AC) \cdot CD

\displaystyle A = \frac{1}{2} (6+ 16) \cdot 15 = \frac{1}{2} (22) \cdot 15 = 165

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