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Review of Dr. Math® Introduces GEOMETRY and
Introduction to Two-Dimensional Geometric Figures not only shows the figures but concentrates on helping the student learn 'n' remember all those difficult terms and words in geometry (obtuse, reflex, scalene, supplemental angles, alternate exterior angles etc.). They become so much easier when you know where they come from. For example, isosceles comes from 'iso-' and 'skelos' and means same-legged. Dr. Math also presents mnemonic tricks to remember your supplemental/complementary angles, or your 45-45-90 and 30-60-90 triangles.
The chapter (as do all) ends with web links where you can learn more, or explore and investigate with interactive models.
Areas and Perimeters of Two-Dimensional Geometric Figures explains problems that involve both area and perimeter. I was delighted at the answer why increasing perimeter doesn't necessarily increase the area, or the visual proof of the area formula for trapezoid. (I always enjoy learning new proofs.) Also explained are some confusing points, such as the difference between length times width and base times height, or 'meters squared' versus 'square meters'.
The third chapter, Circles and Pi first goes to the nitty-gritty of radius and diameter - you may not have thought of these two this profoundly (We actually use the word 'radius' to mean two different things). Dr. Math is not afraid to use common language such as "pie are square" alongside A = πr2. There is a proof for the area formula, and an excellent question on philosophical lines: how could one length of something be measured as an exact number and another not (relating to the fact either radius or circumference must be irrational)? School mathematics does not always explain such things.
Next, Introduction to Three-Dimensional (3-D) Geometric Figures we first learn the basics of polyhedra and platonic solids. The section on surface area emphasizes how to 'figure it out' without memorizing formulas. It also includes a very important question about 2 cups (of rice) and cubic inches both being a measure of volume. Also touched on is the topic of nets of solids.
Symmetry deals with geometrical transformations, different types of symmetry, and briefly on tessellations. Lots of pictures are included; yet this section felt more like being on an introductory level.
The first part, Points, Lines, Planes, Angles, and Their Relationships talks about parallel lines and angles, gives a short explanation of non-Euclidean geometries, and touches on the distance formula and coordinate system.
The part Logic and Proof gives numerous examples of how to build two-column proofs, and tackles many common questions and misunderstandings and difficulties students typically have about proving.
Dr. Math explains how to think of the proof as a 'bridge', how to get started, how mathematicians often build the proof 'backwards' and only later write it neatly, etc. I want to emphasize that the examples here aren't just the proofs themselves but include the thought processes of how a math doctor finds the arguments that make up the proof, and then how it is later written down into the rigid two-column format. Excellent chapter.
There is a lot to learn about triangles, and the part Triangles: Properties, Congruence and Similarity again presents an excellent collection of questions-answers to cover these things. I was delighted at the style - it is like a teacher talking personally to you, sometimes not giving you quite all the answers, while still explaining the basics of how the answer is found.
The fourth part has to do with Quadrilaterals and other Polygons. You learn about interior and outerior angles of polygons, diagonals and heights, area and perimeter.
The problems here are very varied: what happens to the area and perimeter if a rectangle is 'pressed on its corners', or if a parallelogram is 'tilted', proving diagonals perpendicular, or proving diagonals of a trapezoid congruent etc. And there is help for learning the areas of different shapes - how to derive the formulas instead of memorizing all of them.
Lastly, Circles and Their Parts. Tangent problems seem so easy with the diagrams and explanations of Dr. Math. The connection between the perpendicular bisector of a chord and the center of circle is used and made clear in several problems. A few proofs here are just a bit more challenging, for example the construction a line tangent to two circles, or the problem involving "power of a point" concept.
All in all, these geometry companion books are packed full of excellent easy-reading explanations and are WELL worth their low price (around $8-$10). Highly recommended.
Review by Maria Miller, MSc, author of HomeschoolMath.net