ISEE Upper Level Math : How to find an angle in other polygons

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

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Example Questions

Example Question #11 : How To Find An Angle In Other Polygons

The measures of the angles of a ten-sided polygon, or decagon, form an arithmetic sequence. The least of the ten degree measures is \(\displaystyle 100 ^{\circ }\). What is the greatest of the ten degree measures?

Possible Answers:

This polygon cannot exist.

\(\displaystyle 160^{\circ}\)

\(\displaystyle 170^{\circ}\)

\(\displaystyle 116^{\circ}\)

\(\displaystyle 140^{\circ}\)

Correct answer:

This polygon cannot exist.

Explanation:

The total of the degree measures of any ten-sided polygon is

\(\displaystyle \left (10-2 \right )180^{\circ} =1,440^{\circ}\).

In an arithmetic sequence, the terms are separated by a common difference, which we will call \(\displaystyle d\). Since the least of the degree measures is \(\displaystyle 100^{\circ }\), the measures of the angles are

\(\displaystyle 100, 100+d, 100+2d, ...100+9d\)

Their sum is 

\(\displaystyle 100+ \left (100+d \right )+\left (100+2d \right ) ...\left (100+9d \right )=1,440\)

\(\displaystyle 1,000+ 45d =1,440\)

\(\displaystyle 45d = 440\)

\(\displaystyle d = 9\frac{7}{9}\)

The greatest of the angle measures is 

\(\displaystyle 100+9d = 100 + 9\left ( 9\frac{7}{9}\right )= 100+88 = 188\)

However, an angle measure cannot exceed \(\displaystyle 180^{\circ}\). The correct choice is that this polygon cannot exist.

Example Question #21 : Geometry

Heptagon

The seven-sided polygon - or heptagon - in the above diagram is regular. What is the measure of \(\displaystyle \angle 1\)?

Possible Answers:

\(\displaystyle 77\frac{1}{7}^{\circ }\)

\(\displaystyle 70^{\circ}\)

\(\displaystyle 38\frac{4}{7}^{\circ }\)

\(\displaystyle 51\frac{3}{7}^{\circ }\)

\(\displaystyle 60 ^{\circ}\)

Correct answer:

\(\displaystyle 51\frac{3}{7}^{\circ }\)

Explanation:

In the diagram below, some other angles have been numbered for the sake of convenience.

Heptagon

An interior angle of a regular heptagon has measure 

\(\displaystyle \frac{(7-2)180^{\circ }}{7}= 128\frac{4}{7}^{\circ }\).

This is the measure of \(\displaystyle \angle 2\).

As a result of the Isosceles Triangle Theorem, \(\displaystyle \angle 3 \cong \angle 4\), so

\(\displaystyle m \angle 3 = \frac{180^{\circ} - m \angle 2}{2} = \frac{180^{\circ} - 128\frac{4}{7}^{\circ }}{2}= 25\frac{5}{7}^{\circ }\).

This is also the measure of \(\displaystyle \angle 5\).

By angle addition, 

\(\displaystyle m\angle 6 = 128\frac{4}{7}^{\circ } - 2 \cdot 25\frac{5}{7}^{\circ } = 77\frac{1}{7}^{\circ }\)

Again, as a result of the Isosceles Triangle Theorem, \(\displaystyle \angle 1 \cong \angle 7\), so

\(\displaystyle m \angle 1 = \frac{180^{\circ} - m \angle 6}{2} = \frac{180^{\circ} - 77\frac{1}{7}^{\circ }^{\circ }}{2}= 51\frac{3}{7}^{\circ }\)

Example Question #11 : How To Find An Angle In Other Polygons

What is the sum of all the interior angles of a decagon (a polygon with ten sides)?

Possible Answers:

\(\displaystyle 1,250\)

\(\displaystyle 1,800\)

\(\displaystyle 1,300\)

\(\displaystyle 1,440\)

Correct answer:

\(\displaystyle 1,440\)

Explanation:

The sum of the angles in a polygon can be found using the equation below, in which t is equal to the total sum of the angles, and n is equal to the number of sides. 

\(\displaystyle t=(n-2)(180)\)

\(\displaystyle t=(10-2)(180)\)

\(\displaystyle t=8\cdot 180\)

\(\displaystyle t=1,440\)

Example Question #12 : Other Polygons

If each angle in a pentagon is equal to \(\displaystyle x\), what is the value of \(\displaystyle \frac{1}{2}x\)?

Possible Answers:

\(\displaystyle 60\)

\(\displaystyle 54\)

\(\displaystyle 540\)

\(\displaystyle 108\)

Correct answer:

\(\displaystyle 54\)

Explanation:

The sum of the angles in a polygon can be found using the equation below, in which t is equal to the total sum of the angles, and n is equal to the number of sides. 

\(\displaystyle t=(5-2)(180)\)

Given that a hexagon has 6 angles, the total number of angles will be:

\(\displaystyle t=(5-2)(180)\)

\(\displaystyle t=3\cdot 180\)

\(\displaystyle t=540\)

To find the value of each angle, we divide 540 by 5. This results in 108 degrees. 

Thus, 

\(\displaystyle x=108\)

\(\displaystyle \frac{1}{2}x=54\)

Example Question #11 : How To Find An Angle In Other Polygons

What is the value of an angle (to the nearest degree) in a polygon with \(\displaystyle 22\) sides if all the angles are equal to one another?

Possible Answers:

\(\displaystyle 3,600^\circ\)

\(\displaystyle 163^\circ\)

\(\displaystyle 180^\circ\)

\(\displaystyle 164^\circ\)

Correct answer:

\(\displaystyle 164^\circ\)

Explanation:

The sum of the angles in a polygon can be found using the equation below, in which t is equal to the total sum of the angles, and n is equal to the number of sides. 

\(\displaystyle t=(n-2)(180)\)

Given that a hexagon has 6 angles, the total number of angles will be:

\(\displaystyle t=(22-2)(180)\)

\(\displaystyle t=20\cdot 180\)

\(\displaystyle t=3,600\)

Given that there are 3,600 degrees total in a polygon with 22 sides, the number of degrees in each angle can be found by dividing 3,600 by 22. To the nearest degree, this results in 164 degrees. Therefore, 164 is the correct answer.   

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