SAT Math : How to find an angle in a polygon

Study concepts, example questions & explanations for SAT Math

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Example Questions

Example Question #372 : Geometry

To the nearest whole degree, give the measure of each interior angle of a regular polygon with 17 sides.

Possible Answers:

\(\displaystyle 174^{\circ }\)

\(\displaystyle 164^{\circ }\)

\(\displaystyle 154^{\circ }\)

\(\displaystyle 169^{\circ }\)

\(\displaystyle 159^{\circ }\)

Correct answer:

\(\displaystyle 159^{\circ }\)

Explanation:

The measure of each interior angle of an \(\displaystyle N\)-sided polygon can be calculated using the formula

\(\displaystyle m = \frac{(N-2)180^{\circ }}{N}\)

Setting \(\displaystyle N = 17\):

\(\displaystyle m = \frac{(17-2)180^{\circ }}{17} = \frac{(15)180^{\circ }}{17} = \frac{2,700^{\circ }}{17} = 158 \frac{14}{17}^{\circ }\)

The correct choice is therefore \(\displaystyle 159^{\circ }\).

Example Question #373 : Geometry

Each interior angle of a regular polygon has measure \(\displaystyle 175 ^{\circ }\). How many sides does the polygon have?

Possible Answers:

\(\displaystyle 108\)

\(\displaystyle 144\)

\(\displaystyle 72\)

\(\displaystyle 36\)

\(\displaystyle 18\)

Correct answer:

\(\displaystyle 72\)

Explanation:

The easiest way to work this is arguably to examine the exterior angles, each of which forms a linear pair with an interior angle. If an interior angle measures \(\displaystyle 175 ^{\circ }\), then each exterior angle, which is supplementary to an interior angle, measures

\(\displaystyle 180 ^{\circ }- 175 ^{\circ } = 5^{\circ }\)

The measures of the exterior angles of a polygon, one per vertex, total \(\displaystyle 360 ^{\circ }\); in a regular polygon, they are congruent, so if there are \(\displaystyle N\) such angles, each measures \(\displaystyle \frac{360^{\circ }}{N}\). Since the number of vertices is equal to the number of sides, if we set this equal to \(\displaystyle 5^{\circ }\) and solve for \(\displaystyle N\), we will find the number of sides.

\(\displaystyle \frac{360 }{N} = 5\)

Multiply both sides by \(\displaystyle \frac{N}{5}\):

\(\displaystyle \frac{360 }{N}\cdot \frac{N}{5}= 5 \cdot \frac{N}{5}\)

\(\displaystyle \frac{360 }{ 5}= N\)

\(\displaystyle 72 = N\)

The polygon has 72 vertices and, thus, 72 sides.

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

A regular polygon has a measure of \(\displaystyle 140^\circ\) for each of its internal angles.  How many sides does it have?

 

Possible Answers:

\(\displaystyle 6\)

\(\displaystyle 7\)

\(\displaystyle 8\)

\(\displaystyle 9\)

Correct answer:

\(\displaystyle 9\)

Explanation:

To determine the measure of the angles of a regular polygon use:

Angle = (n – 2) x 180° / n

Thus, (n – 2) x 180° / n = 140°

180° n - 360° = 140° n

40° n = 360°

n = 360° / 40° = 9

Example Question #11 : How To Find An Angle In A Polygon

A regular seven sided polygon has a side length of 14”.  What is the measurement of one of the interior angles of the polygon?

Possible Answers:

257.14 degrees

252 degrees

154.28 degrees

180 degrees

128.57 degrees

Correct answer:

128.57 degrees

Explanation:

The formula for of interior angles based on a polygon with a number of side n is:

Each Interior  Angle = (n-2)*180/n

= (7-2)*180/7 = 128.57 degrees

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