PSAT Math : Plane Geometry

Study concepts, example questions & explanations for PSAT Math

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Example Questions

Example Question #1 : How To Find An Angle In A Hexagon

What is the measurement of one of the interior angles of a regular hexagon?

Possible Answers:

\displaystyle 150^\circ

\displaystyle 120^\circ

\displaystyle 105^\circ

\displaystyle 135^\circ

\displaystyle 90^\circ

Correct answer:

\displaystyle 120^\circ

Explanation:

To find the sum of the interior angles of any regular polygon, use the formula \displaystyle (n-2)\cdot180, where \displaystyle n represents the number of sides of the regular polygon.

\displaystyle =(6-2)\cdot180

\displaystyle =4\cdot180

\displaystyle =720

The sum of the interior angles of a regular hexagon is 720 degrees. To find the measurement of one angle, divide by the number of interior angles (or sides):

\displaystyle =720/6

\displaystyle =120

The measurement of one angle in a regular hexagon is 120 degrees.

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

Calculate the approximate area a regular hexagon with the following side length:

\displaystyle s=6

Possible Answers:

\displaystyle 98

Cannot be determined 

\displaystyle 94

\displaystyle 88

\displaystyle 100

Correct answer:

\displaystyle 94

Explanation:

How do you find the area of a hexagon?

There are several ways to find the area of a hexagon.

  1. In a regular hexagon, split the figure into triangles.
  2. Find the area of one triangle.
  3. Multiply this value by six.

Alternatively, the area can be found by calculating one-half of the side length times the apothem.

 

Regular hexagons:

Regular hexagons are interesting polygons. Hexagons are six sided figures and possess the following shape:

In a regular hexagon, all sides equal the same length and all interior angles have the same measure; therefore, we can write the following expression.

\displaystyle \overline{AB}=\overline{BC}=\overline{CD}=\overline{DE}=\overline{EF}=\overline{FA} 

One of the easiest methods that can be used to find the area of a polygon is to split the figure into triangles. Let's start by splitting the hexagon into six triangles. 

Screen shot 2016 07 06 at 2.09.44 pm

In this figure, the center point, \displaystyle X, is equidistant from all of the vertices. As a result, the six dotted lines within the hexagon are the same length. Likewise, all of the triangles within the hexagon are congruent by the side-side-side rule: each of the triangle's share two sides inside the hexagon as well as a base side that makes up the perimeter of the hexagon. In a similar fashion, each of the triangles have the same angles. There are \displaystyle 360^{\circ} in a circle and the hexagon in our image has separated it into six equal parts; therefore, we can write the following:

Screen shot 2016 07 06 at 2.27.41 pm

\displaystyle \theta=\frac{360^{\circ}}{6}

\displaystyle \theta=60^{\circ

We also know the following:

\displaystyle \angle AXB=\angle BXC=\angle CXD=\angle DXE=\angle EXF=\angle FXA=60^{\circ}

Now, let's look at each of the triangles in the hexagon. We know that each triangle has two two sides that are equal; therefore, each of the base angles of each triangle must be the same. We know that a triangle has \displaystyle 180^{\circ} and we can solve for the two base angles of each triangle using this information.

\displaystyle 2x+60^{\circ}=180^{\circ}

\displaystyle 2x+=120^{\circ}

\displaystyle x=60^{\circ}

Each angle in the triangle equals \displaystyle 60^{\circ}. We now know that all the triangles are congruent and equilateral: each triangle has three equal side lengths and three equal angles. Now, we can use this vital information to solve for the hexagon's area. If we find the area of one of the triangles, then we can multiply it by six in order to calculate the area of the entire figure. Let's start by analyzing \displaystyle \triangle BXC. If we draw, an altitude through the triangle, then we find that we create two \displaystyle 30^{\circ}-60^{\circ}-90^{\circ} triangles. 

Screen shot 2016 07 06 at 2.27.10 pm

Let's solve for the length of this triangle. Remember that in \displaystyle 30^{\circ}-60^{\circ}-90^{\circ} triangles, triangles possess side lengths in the following ratio:

\displaystyle 1:2:\sqrt{3}

Now, we can analyze \displaystyle \triangle BXC using the a substitute variable for side length, \displaystyle s.

Screen shot 2016 07 06 at 3.01.03 pm

We know the measure of both the base and height of \displaystyle \triangle BXC and we can solve for its area.

\displaystyle \textup{Area of }\triangle BXC=\frac{1}{2}\times \textup{base}\times \textup{height}

\displaystyle \textup{Area of }\triangle BXC=\frac{1}{2}\times s\times \frac{\sqrt{3}s}{2}

\displaystyle \textup{Area of }\triangle BXC=\frac{\sqrt{3}s^2}{4}

Now, we need to multiply this by six in order to find the area of the entire hexagon.

\displaystyle \textup{Area of Hexagon}[ABCDEF]=6\times\frac{\sqrt{3}s^2}{4}

\displaystyle \textup{Area of Hexagon}[ABCDEF]=\frac{6 \sqrt{3}s^2}{4}

\displaystyle \textup{Area of Hexagon}[ABCDEF]=\frac{3\sqrt{3}}{2}\times s^2

We have solved for the area of a regular hexagon with side length, \displaystyle s. If we know the side length of a regular hexagon, then we can solve for the area.

If we are not given a regular hexagon, then we an solve for the area of the hexagon by using the side length(i.e. \displaystyle s) and apothem (i.e. \displaystyle a), which is the length of a line drawn from the center of the polygon to the right angle of any side. This is denoted by the variable \displaystyle a in the following figure:

Screen shot 2016 07 06 at 3.17.05 pm

 

Alternative method:

If we are given the variables \displaystyle s and \displaystyle a, then we can solve for the area of the hexagon through the following formula:

\displaystyle A=\frac{1}{2}(P)(a)

In this equation, \displaystyle A is the area, \displaystyle P is the perimeter, and \displaystyle a is the apothem. We must calculate the perimeter using the side length and the equation \displaystyle P=\textup{number of sides}\times s, where \displaystyle s is the side length.

 

Solution:

In the given problem we know that the side length of a regular hexagon is the following:

\displaystyle s=6

Let's substitute this value into the area formula for a regular hexagon and solve.

\displaystyle A=\frac{3\sqrt{3}}{2}\times s^2

\displaystyle A=\frac{3\sqrt{3}}{2}\times 6^2

Simplify.

\displaystyle A=\frac{108\sqrt{3}}{2}

\displaystyle A=54\sqrt{3}

\displaystyle A=93.53

Round the answer to the nearest whole number.

\displaystyle A=94

Example Question #1 : How To Find An Angle In A Pentagon

What is the measure, in degrees, of one interior angle of a regular pentagon?

Possible Answers:

144

72

108

180

120

Correct answer:

108

Explanation:

The formula for the sum of the interior angles of any regular polygon is as follows:

\displaystyle \small = 180(n-2)

where \displaystyle \small n is equal to the number of sides of the regular polygon.

Therefore, the sum of the interior angles for a regular pentagon is:

\displaystyle \small =180(5-2)

\displaystyle \small =180(3)

\displaystyle \small =540

To find the measure of one interior angle of a regular pentagon, simply divide by the number of sides (or number of interior angles):

\displaystyle \small =\frac{540}{5}

\displaystyle \small =108

The measure of one interior angle of a regular pentagon is 108 degrees.

Example Question #1 : How To Find An Angle In A Pentagon

Pentagon_1

 

Refer to the above figure, which shows Square \displaystyle OPQK  and regular Pentagon \displaystyle KLMNO.

Evaluate \displaystyle m \angle LOQ.

Possible Answers:

\displaystyle 81^{\circ }

\displaystyle 67\frac{1}{2} ^{\circ }

\displaystyle 90 ^{\circ }

\displaystyle 75 ^{\circ }

\displaystyle 72 ^{\circ }

Correct answer:

\displaystyle 81^{\circ }

Explanation:

By angle addition,

\displaystyle m \angle LOQ = m \angle LOK + m \angle KOQ

 

\displaystyle \angle KOQ is one of two acute angles of isosceles right triangle \displaystyle \Delta KOQ, so \displaystyle m \angle KOQ = 45^{\circ }.

 

To find \displaystyle m \angle LOK we examine \displaystyle \Delta LOK.

\displaystyle \angle LKO is an angle of a regular pentagon and has measure \displaystyle \frac{180^{\circ}\times (5-2)}{5} = 108 ^{\circ}.

Also, since, in \displaystyle \Delta LOK, sides \displaystyle \overline{KL} \cong \overline{KO}, by the Isosceles Triangle Theorem, \displaystyle m \angle LOK = m \angle KLO

Since the angles of a triangle must total \displaystyle 180^{\circ } in measure, 

\displaystyle m \angle LOK + m \angle KLO + m \angle LKO = 180^{\circ }

\displaystyle m \angle LOK + m \angle LOK + 108 ^{\circ } = 180^{\circ }

\displaystyle 2 m \angle LOK + 108 ^{\circ } = 180^{\circ }

\displaystyle 2 m \angle LOK = 72^{\circ }

\displaystyle m \angle LOK = 36^{\circ }

 

\displaystyle m \angle LOQ = m \angle LOK + m \angle KOQ

\displaystyle m \angle LOQ = 36^{\circ } + 45 ^{\circ } = 81^{\circ }

Example Question #591 : Psat Mathematics

Pythagoras

Refer to the above figure. You are given that \displaystyle AB = 5 and \displaystyle BC = 12. Which of the following statements would be sufficient to prove that \displaystyle \angle 2 is a right angle, given what is already known?

I) \displaystyle AC = 13

II) \displaystyle \angle BAC and \displaystyle \angle BCA are both acute

III) \displaystyle \angle 1 is a right angle

Possible Answers:

I, II, and III

I and II only

None of these

I and III only

II and III only

Correct answer:

I and III only

Explanation:

If \displaystyle AC = 13, then \displaystyle \Delta ABC has short sides \displaystyle AB = 5 , BC = 12 and long side \displaystyle AC = 13. Since

\displaystyle 5^{2} + 12^{2} = 25 + 144 = 169 = 13^{2},

then, by the converse of the Pythagorean Theorem, \displaystyle \Delta ABC is a right triangle with right angle \displaystyle \angle 2. Statement I is sufficient.

 

If \displaystyle \angle BAC and \displaystyle \angle BCA are both acute,we know nothing about \displaystyle \angle 2; every triangle has at least two acute angles regardless of type. Statement II tells us nothing.

 

\displaystyle \angle 1 and \displaystyle \angle 2 form a linear pair and are therefore supplementary. If one is a right angle, so is the other. Therefore, if  \displaystyle \angle 1 is a right angle, so is \displaystyle \angle 2. Statement III is sufficient.

 

The correct response is Statement I and III only.

Example Question #2 : Lines

Thingy_5

Note: \displaystyle m\parallel n

Refer to the above diagram. \displaystyle \angle DCG and which other angle form a pair of corresponding angles?

Possible Answers:

\displaystyle \angle ACB

\displaystyle \angle HGJ

\displaystyle \angle BCG

\displaystyle \angle CGH

\displaystyle \angle CGF 

Correct answer:

\displaystyle \angle HGJ

Explanation:

Two angles formed by a transversal line crossing two other lines are corresponding angles if, relative to the points of intersection, they are in the same position. \displaystyle \angle DCG is formed by the intersection of transversal \displaystyle t and \displaystyle m; the angle in the same relative position where \displaystyle t intersects \displaystyle n is \displaystyle \angle HGJ.

Example Question #591 : Psat Mathematics

Thingy_5

Refer to the above diagram. \displaystyle \angle DCG and which other angle form a pair of alternate interior angles?

Possible Answers:

\displaystyle \angle CGF 

\displaystyle \angle BCG

\displaystyle \angle CGH

\displaystyle \angle HGJ

\displaystyle \angle ACB

Correct answer:

\displaystyle \angle CGF 

Explanation:

Two angles formed by a transversal line crossing two other lines are alternate interior angles if:

I) Both angles have their interiors between the lines crossed

II) The angles have their interiors on the opposite sides of the transversal.

Of the given choices, only \displaystyle \angle CGF fits the description; the interior of each is between \displaystyle m and \displaystyle n, and the interiors are on the opposite sides of \displaystyle t.

Example Question #1 : How To Find The Angle Of Two Lines

Thingy_5

Refer to the above diagram. 

\displaystyle \angle ABC and which other angle form a pair of vertical angles?

Possible Answers:

\displaystyle \angle BFG

\displaystyle \angle ZBF

\displaystyle \angle ACB

\displaystyle \angle ZBA

\displaystyle \angle AFY

Correct answer:

\displaystyle \angle ZBF

Explanation:

Two angles are vertical angles if they share a vertex, anf if their union is a pair of intersecting lines. Of the five choices, only \displaystyle \angle ZBF fits both descriptions with \displaystyle \angle ABC.

Example Question #1 : How To Find A Ray

Thingy

Refer to the above figure.

Which of the following segments is a diagonal of Pentagon \displaystyle MNOQK ?

Possible Answers:

\displaystyle \overline{QK}

\displaystyle \overline{RN}

\displaystyle \overline{OK}

\displaystyle \overline{ML}

\displaystyle \overline{OR}

Correct answer:

\displaystyle \overline{OK}

Explanation:

A diagonal of a polygon is a segment whose endpoints are nonconsecutive vertices of the polygon. Of the five choices, only \displaystyle \overline{OK} fits this description.

Example Question #1 : Rhombuses

If the area of a rhombus is 24 and one diagonal length is 6, find the perimeter of the rhombus.

Possible Answers:

8

12

16

24

20

Correct answer:

20

Explanation:

The area of a rhombus is found by

A = 1/2(d1)(d2)

where d1 and d2 are the lengths of the diagonals.  Substituting for the given values yields

24 = 1/2(d1)(6)

24 = 3(d1)

8 = d1

Now, use the facts that diagonals are perpendicular in a rhombus, diagonals bisect each other in a rhombus, and the Pythagorean Theorem to determine that the two diagonals form 4 right triangles with leg lengths of 3 and 4.  Since 32 + 42 = 52, each side length is 5, so the perimeter is 5(4) = 20.

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