Geometry For Dummies

By: Mark Ryan Published: 07-05-2016

Hit the geometry wall? Get up and running with this no-nonsense guide!

Does the thought of geometry make you jittery? You're not alone. Fortunately, this down-to-earth guide helps you approach it from a new angle, making it easier than ever to conquer your fears and score your highest in geometry. From getting started with geometry basics to making friends with lines and angles, you'll be proving triangles congruent, calculating circumference, using formulas, and serving up pi in no time.

Geometry is a subject full of mathematical richness and beauty. But it's a subject that bewilders many students because it's so unlike the math they've done before—it requires the use of deductive logic in formal proofs. If you're having a hard time wrapping your mind around what that even means, you've come to the right place! Inside, you'll find out how a proof's chain of logic works and even discover some secrets for getting past rough spots along the way. You don't have to be a math genius to grasp geometry, and this book helps you get un-stumped in a hurry!

• Find out how to decode complex geometry proofs
• Learn to reason deductively and inductively
• Make sense of angles, arcs, area, and more

There's no reason to let your nerves get jangled over geometry—your understanding will take new shape with the help of Geometry For Dummies.

Articles From Geometry For Dummies

59 results
59 results
Geometry For Dummies Cheat Sheet

Cheat Sheet / Updated 02-08-2022

Successfully understanding and studying geometry involves using strategies for your geometry proofs, knowing important equations, and being able to identify commonly used geometry symbols.

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Measuring and Making Angles

Article / Updated 12-21-2021

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How to Identify Arcs, Central Angles, and Lengths of Arcs

Article / Updated 09-17-2021

A circle's central angles and the arcs that they cut out are part of many circle proofs. They also come up in many area problems. The following figure shows how an angle and an arc are interrelated. Arc: An arc is simply a curved piece of a circle. Any two points on a circle divide the circle into two arcs: a minor arc (the smaller piece) and a major arc (the larger)—unless the points are the endpoints of a diameter, in which case both arcs are semicircles. Note that to name a minor arc, you use its two endpoints; to name a major arc, you use its two endpoints plus any point along the arc. Central angle: A central angle is an angle whose vertex is at the center of a circle. The two sides of a central angle are radii that hit the circle at the opposite ends of an arc—or as mathematicians say, the angle intercepts the arc. The measure of an arc is the same as the degree measure of the central angle that intercepts it. Determining the length of an arc An arc’s length means the same commonsense thing length always means — you know, like the length of a piece of string (with an arc, of course, it’d be a curved piece of string). Make sure you don’t mix up arc length with the measure of an arc which is the degree size of its central angle. A circle is 360° all the way around; therefore, if you divide an arc’s degree measure by 360°, you find the fraction of the circle’s circumference that the arc makes up. Then, if you multiply the length all the way around the circle (the circle’s circumference) by that fraction, you get the length along the arc. So finally, here’s the formula you’ve been waiting for. Arc length: Its degree measure is 45° and the radius of the circle is 12, so here’s the math for its length: Pretty simple, eh?

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How to Prove a Quadrilateral Is a Parallelogram

Article / Updated 07-12-2021

There are five ways in which you can prove that a quadrilateral is a parallelogram. The first four are the converses of parallelogram properties (including the definition of a parallelogram). Make sure you remember the oddball fifth one — which isn’t the converse of a property — because it often comes in handy: If both pairs of opposite sides of a quadrilateral are parallel, then it’s a parallelogram (reverse of the definition). If both pairs of opposite sides of a quadrilateral are congruent, then it’s a parallelogram (converse of a property). Tip: To get a feel for why this proof method works, take two toothpicks and two pens or pencils of the same length and put them all together tip-to-tip; create a closed figure, with the toothpicks opposite each other. The only shape you can make is a parallelogram. If both pairs of opposite angles of a quadrilateral are congruent, then it’s a parallelogram (converse of a property). If the diagonals of a quadrilateral bisect each other, then it’s a parallelogram (converse of a property). Tip: Take, say, a pencil and a toothpick (or two pens or pencils of different lengths) and make them cross each other at their midpoints. No matter how you change the angle they make, their tips form a parallelogram. If one pair of opposite sides of a quadrilateral are both parallel and congruent, then it’s a parallelogram (neither the reverse of the definition nor the converse of a property). Tip: Take two pens or pencils of the same length, holding one in each hand. If you keep them parallel, no matter how you move them around, you can see that their four ends form a parallelogram. The preceding list contains the converses of four of the five parallelogram properties. If you’re wondering why the converse of the fifth property (consecutive angles are supplementary) isn’t on the list, you have a good mind for details. The explanation, essentially, is that the converse of this property, while true, is difficult to use, and you can always use one of the other methods instead.

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The Properties of Trapezoids and Isosceles Trapezoids

Article / Updated 07-09-2021

A trapezoid is a quadrilateral (a shape with four sides) with exactly one pair of parallel sides (the parallel sides are called bases). The following figure shows a trapezoid to the left, and an isosceles trapezoid on the right. The properties of the trapezoid are as follows: The bases are parallel by definition. Each lower base angle is supplementary to the upper base angle on the same side. The properties of the isosceles trapezoid are as follows: The properties of a trapezoid apply by definition (parallel bases). The legs are congruent by definition. The lower base angles are congruent. The upper base angles are congruent. Any lower base angle is supplementary to any upper base angle. The diagonals are congruent. The supplementary angles might be the hardest property to spot in the diagrams above. Because of the parallel sides, consecutive angles are same-side interior angles and are thus supplementary. (All the special quadrilaterals except the kite, by the way, contain consecutive supplementary angles.) Here’s an isosceles trapezoid proof for you: Statement 1: Reason for statement 1: Given. Statement 2: Reason for statement 2: The legs of an isosceles trapezoid are congruent. Statement 3: Reason for statement 3: The upper base angles of an isosceles trapezoid are congruent. Statement 4: Reason for statement 4: Reflexive Property. Statement 5: Reason for statement 5: Side-Angle-Side, or SAS (2, 3, 4) Statement 6: Reason for statement 6: CPCTC (Corresponding Parts of Congruent Triangles are Congruent). Statement 7: Reason for statement 7: If angles are congruent, then so are sides.

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Interior and Exterior Angles of a Polygon

Article / Updated 07-08-2021

Everything you need to know about a polygon doesn’t necessarily fall within its sides. You may need to find exterior angles as well as interior angles when working with polygons: Interior angle: An interior angle of a polygon is an angle inside the polygon at one of its vertices. Angle Q is an interior angle of quadrilateral QUAD. Exterior angle: An exterior angle of a polygon is an angle outside the polygon formed by one of its sides and the extension of an adjacent side. Interior and exterior angle formulas: The sum of the measures of the interior angles of a polygon with n sides is (n – 2)180. The measure of each interior angle of an equiangular n-gon is If you count one exterior angle at each vertex, the sum of the measures of the exterior angles of a polygon is always 360°. Check here for more practice.

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How to Calculate the Area of a Regular Octagon

Article / Updated 07-07-2021

You can calculate the area of a regular octagon with the standard regular polygon method, but there’s a nifty alternative method based on the fact that a regular octagon is a square with its four corners cut off. For example, here’s how you’d find the area of EIGHTPLU in the figure below, given that it’s a regular octagon with sides of length 6. The four corners (like triangle SUE in the figure) that you cut off the square to turn it into an octagon are 45°- 45°- 90° triangles (you can prove that to yourself if you feel like it). So all you have to do to get the area of the octagon is to calculate the area of the square and then subtract the four corner triangles. Piece o’ cake. But first, here are two great tips for this and other problems. For problems involving regular octagons, 45°- 45°- 90° triangles can come in handy. Add segments to the diagram to get one or more 45°- 45°- 90° triangles and some squares and rectangles to help you solve the problem. Think outside the box. It’s easy to get into the habit of looking only inside a figure because that suffices for the vast majority of problems. But occasionally (like in this problem), you need to break out of that rut and look outside the perimeter of the figure. Okay, so here’s what you do. You’re given that the octagon’s sides have a length of 6. Consider side EU. Not just calculate the area of the square and the area of a single corner triangle: To finish, subtract the total area of the four corner triangles from the area of the square:

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How to Identify Radii, Chords, and Diameters

Article / Updated 07-07-2021

When you work with circles, there are three straight-line components that you need to be able to identify: radii, chords, and diameters. Radius: A circle’s radius — the distance from its center to a point on the circle — tells you the circle’s size. In addition to being a measure of distance, a radius is also a segment that goes from a circle’s center to a point on the circle. Chord: A segment that connects two points on a circle is called a chord. Diameter: A chord that passes through a circle’s center is a diameter of the circle. A circle’s diameter is twice as long as its radius. The above figure shows circle O.

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How to Copy an Angle Using a Compass

Article / Updated 12-08-2016

The basic idea behind copying a given angle is to use your compass to sort of measure how wide the angle is open; then you create another angle with the same amount of opening. Here's the proof diagram. Refer to the figure as you work through these steps: Draw a working line, l, with point B on it. Open your compass to any radius r, and construct arc (A, r) intersecting the two sides of angle A at points S and T. Construct arc (B, r) intersecting line l at some point V. Construct arc (S, ST). Construct arc (V, ST) intersecting arc (B, r) at point W. Draw line BW and you're done.

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How to Copy a Line Segment Using a Compass

Article / Updated 12-08-2016

The key to copying a given line segment is to open your compass to the length of the segment; then, using that amount of opening, you can mark off another segment of the same length. Here's the proof diagram. This figure shows the solution; refer to it as you work through the following steps. Using your straightedge, draw a working line, l, with a point P anywhere on it. Put your compass point on point M and open it to the length of line MN. The best way to make sure you've opened it to just the right amount is to draw a little arc that passes through N. In other words, draw arc (M, MN). Being careful not to change the amount of the compass's opening from Step 2, put the compass point on point P and construct arc (P, MN) intersecting line l. You call this point of intersection point Q, and you're done.

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