PSAT Math : Other Polygons

Study concepts, example questions & explanations for PSAT Math

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Example Questions

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

In isosceles triangle ABC, the measure of angle A is 50 degrees.  Which is NOT a possible measure for angle B?

Possible Answers:

65 degrees

80 degrees

There is more than one correct answer

50 degrees

95  degrees

Correct answer:

95  degrees

Explanation:

If angle A is one of the base angles, then the other base angle must measure 50 degrees. Since 50 + 50 + x = 180 means x = 80, the vertex angle must measure 80 degrees.

If angle A is the vertex angle, the two base angles must be equal. Since 50 + x + x = 180 means x = 65, the two base angles must measure 65 degrees.

The only number given that is not possible is 95 degrees.

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

In triangle ABC, the measure of angle A = 70 degrees, the measure of angle Bx degrees, and the measure of angle Cy degrees. What is the value of y in terms of x?

Possible Answers:

70 – x

110 + x

110 – x

x – 70

70 + x

Correct answer:

110 – x

Explanation:

Since the three angles of a triangle sum to 180, we know that 70 + x + y = 180. Subtract 70 from both sides and see that x + y = 110. Subtract x from both sides and see that y = 110 – x.

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

What is the measure, in degrees, of each interior angle of a regular convex polygon that has twelve sides?

Possible Answers:

120

150

135

175

180

Correct answer:

150

Explanation:

The sum of the interior angles, in degrees, of a regular polygon is given by the formula 180(n – 2), where n is the number of sides. The problem concerns a polygon with twelve sides, so we will let n = 12. The sum of the interior angles in this polygon would be 180(12 – 2) = 180(10) = 1800.

Because the polygon is regular (meaning its sides are all congruent), all of the angles have the same measure. Thus, if we divide the sum of the measures of the angles by the number of sides, we will have the measure of each interior angle. In short, we need to divide 1800 by 12, which gives us 150.

The answer is 150.

Example Question #2 : How To Find An Angle In A Polygon

Octagon

In the figure above, polygon ABDFHGEC is a regular octagon. What is the measure, in degrees, of angle FHI?

Possible Answers:

60

45

40

30

50

Correct answer:

45

Explanation:

Angle FHI is the supplement of angle FHG, which is an interior angle in the octagon. When two angles are supplementary, their sum is equal to 180 degrees. If we can find the measure of each interior angle in the octagon, then we can find the supplement of angle FHG, which will give us the measure of angle FHI.

The sum of the interior angles in a regular polygon is given by the formula 180(n – 2), where n is the number of sides in the polygon. An octagon has eight sides, so the sum of the angles of the octagon is 180(8 – 2) = 180(6) = 1080 degrees. Because the octagon is regular, all of its sides and angles are congruent. Thus, the measure of each angle is equal to the sum of its angles divided by 8. Therefore, each angle in the polygon has a measure of 1080/8 = 135 degrees. This means that angle FHG has a measure of 135 degrees.

Now that we know the measure of angle FHG, we can find the measure of FHI. The sum of the measures of FHG and FHI must be 180 degrees, because the two angles form a line and are supplementary. We can write the following equation:

Measure of FHG + measure of FHI = 180

135 + measure of FHI = 180

Subtract 135 from both sides.

Measure of FHI = 45 degrees.

The answer is 45. 

Example Question #9 : Other Polygons

What is the measure of each angle in a regular octagon?

Possible Answers:

135

90

150

180

75

Correct answer:

135

Explanation:

An octagon contains six triangles, or 1080 degrees. This means with 8 angles, each angle is 135 degrees.

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

What is the measure of each central angle of an octagon?

Possible Answers:

120

90

60

35

45

Correct answer:

45

Explanation:

There are 360 degrees and 8 angles, so dividing leaves 45 degrees per angle.

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

Pentagon

Note: Figure NOT drawn to scale.

Refer to the above figure.   is equilateral and Pentagon  is regular.

Evaluate .

Possible Answers:

Correct answer:

Explanation:

By angle addition, 

 

 is an angle of a reguar pentagon, so its measure is .

 

To find , first we find .

By angle addition, 

 is an angle of a regular pentagon and has measure .

, as an angle of an equilateral triangle, has measure 

 is equilateral, so ; Pentagon  is regular, so . Therefore, , and by the Isosceles Triangle Theorem, .

The degree measures of three angles of a triangle total , so:

 

 

Since

we have 

Example Question #12 : Other Polygons

Pentagon  is regular. If diagonal  is drawn, which of the following describes Quadrilateral ?

Possible Answers:

None of the other responses is correct.

Quadrilateral  is a parallelogram but neither a rectangle nor a rhombus.

Quadrilateral  is a rhombus but not a rectangle.

Quadrilateral  is a trapezoid.

Quadrilateral  is a rectangle but not a rhombus.

Correct answer:

Quadrilateral  is a trapezoid.

Explanation:

The figure described is below.

Pentagon

Each of the angles of the pentagon has measure 

 is an isosceles triangle, and , so 

and

Since

,

and by the parallel postulate,

Quadrilateral  has exactly one pair of parallel sides, so it is a trapezoid. 

Example Question #1 : Other Polygons

If the following shape was going to be drawn in a circle, what is the minimum radius of the circle?

Possible Answers:

10

11

7

8

9

Sat_math_picture3


Correct answer:

7

Explanation:

IF you draw the longest diagonal across the shape, the length of it is 13.4. This means the radius must be at least 6.7. The answer is 7.

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

Heptagon

Note: Figure NOT drawn to scale.

The above polygon has perimeter 190. Evaluate .

Possible Answers:

Correct answer:

Explanation:

To get the expression equivalent to the perimeter, add the lengths of the sides:

Since the perimeter is 190, we can simplify this to

and solve as follows:

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