# Geometry Quick Review: Triangles - Key Theorems and Proofs (Quick Review Notes) This corollary of the previous theorem is a particularly significant result about angles in circles:. The last two theorems are often expressed in slightly different language, and some explanation is needed to avoid confusion. An altitude of a triangle is a perpendicular from any of the three vertices to the opposite side, produced if necessary. The two cases are illustrated in the diagrams below. There are three altitudes in a triangle. The following theorem proves that they concurrent at a point called the orthocentre H of the triangle.

It is surprising that circles can be used to prove the concurrence of the altitudes. The altitudes of a triangle are concurrent. In the module, Congruence , we showed how to draw the circumcircle through the vertices of any triangle. To do this, we showed that the perpendicular bisectors of its three sides are concurrent, and that their intersection, called the circumcentre of the triangle, is equidistant from each vertex.

No other circle passes through these three vertices. If we tried to take as centre a point P other than the circumcentre, then P would not lie on one of the perpendicular bisectors, so it could not be equidistant from the three vertices. When there are four points, we can always draw a circle through any three of them provided they are not collinear , but only in special cases will that circle pass through the fourth point.

A cyclic quadrilateral is a quadrilateral whose vertices all lie on a circle. This is the last type of special quadrilateral that we shall consider. Suppose that we are given a quadrilateral that is known to be cyclic, but whose circumcentre is not shown perhaps it has been rubbed out. The circumcentre of the quadrilateral is the circumcentre of the triangle formed by any three of its vertices, so the construction to the right will find its circumcentre. The distinctive property of a cyclic quadrilateral is that its opposite angles are supplementary.

The following proof uses the theorem that an angle at the circumference is half the angle at the centre standing on the same arc. Join the radii OB and OD. Here is an alternative proof using the fact that two angles in the same segment are equal. An exterior angle of a cyclic quadrilateral is supplementary to the adjacent interior angle, so is equal to the opposite interior angle. This gives us the corollary to the cyclic quadrilateral theorem:. An exterior angle of a cyclic quadrilateral is equal to the opposite interior angle. This exterior angle and A are both supplementary to BCD , so they are equal.

Show that AP CR in the diagram to the right. If a cyclic trapezium is not a rectangle, show that the other two sides are not parallel, but have equal length. The property of a cyclic quadrilateral proven earlier, that its opposite angles are supplementary, is also a test for a quadrilateral to be cyclic. That is the converse is true. If the opposite angles of a quadrilateral are supplementary, then the quadrilateral is cyclic. Construct the circle through A , B and D , and suppose, by way of contradiction, that the circle does not pass through C.

If an exterior angle of a quadrilateral equals the opposite interior angle, then the quadrilateral is cyclic. In the diagram to the right, the two adjacent acute angles of the trapezium are equal. Prove that the trapezium is cyclic.

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The sine rule states that for any triangle ABC , the ratio of any side over the sine of its opposite angle is a constant,. Each term is the ratio of a length over a pure number, so their common value seems to be a length. Thus it reasonable to ask, what is this common length? The proof of this result provides a proof of the sine rule that is independent of the proof given in the module, Further Trigonometry.

It is sufficient to prove that is the diameter of the circumcircle. A tangent to a circle is a line that meets the circle at just one point. The diagram below shows that given a line and a circle, can arise three possibilities:. The point where a tangent touches a circle is called a point of contact. It is not immediately obvious how to draw a tangent at a particular point on a circle, or even whether there may be more than one tangent at that point. Let T be a point on a circle with centre O. First we prove parts a and c.

Let be the line through T perpendicular to the radius OT. Let P be any other point on , and join the interval OP. Hence P lies outside the circle, and not on it. This proves that the line is a tangent, because it meets the circle only at T. It also proves that every point on , except for T , lies outside the circle.

It remains to prove part b , that there is no other tangent to the circle at T. Let t be a tangent at T , and suppose, by way of contradiction, that t were not perpendicular to OT. Hence U also lies on the circle, contradicting the fact that t is a tangent. Using this radius and tangent theorem, and the angle in a semi circle theorem, we can now construct tangents to a circle with centre O from a point P outside the circle.

It is also a simple consequence of the radius-and-tangent theorem that the two tangents PT and PU have equal length. The right angle formed by a radius and tangent gives further opportunities for simple trigonometry. The following exercise involves quadrilaterals within which an incircle can be drawn tangent to all four sides.

These quadrilaterals form yet another class of special quadrilaterals. The sides of a quadrilateral are tangent to a circle drawn inside it. Show that the sums of opposite sides of the quadrilateral are equal. A line that is tangent to two circles is called a common tangent to the circles. When the points of contact are distinct, there are two cases, as in the diagrams below. The two circles lie on the same side of a direct common tangent , and lie on opposite sides of an indirect common tangent. Two circles are said to touch at a common point T if there is a common tangent to both circles at the point T.

As in the diagram below, the circles touch externally when they are on opposite sides of the common tangent, and touch internally when they are on the same side of the common tangent. Provided that they are distinct, touching circles have only the one point in common. We are now in a position to prove a wonderful theorem on the angle bisectors of a triangle. These three bisectors are concurrent, and their point of intersection is called the incentre of the triangle.

The incentre is the centre of the incircle tangent to all three sides of the triangle, as in the diagram to the right. The angle bisectors of a triangle are concurrent, and the resulting incentre is the centre of the incircle, that is tangent to all three sides. This completes the development of the four best-known centres of a triangle. The results, and the associated terminology and notation, are summarised here for reference.

In the next two diagrams, the angle BQU remains equal to P as the point Q moves around the arc closer and closer to A. In the last diagram, Q coincides with A , and AU is a tangent. An angle between a chord and a tangent is equal to any angle in the alternate segment. We have already proven that all the angles in this left-hand segment are equal. There are two equally satisfactory proofs of this theorem. One is written out below and the other is left as an exercise.

The centre O lies on the chord AB. The centre O lies outside the arms of ABU. The centre O lies within the arms of ABU. Find an alternative proof in cases 2 and 3 by constructing the radii AO and BO and using angles at the centre. The result in the following exercise is surprising. One would not expect parallel lines to emerge so easily in a diagram with two touching circles. The final theorems in this module combine similarity with circle geometry to produce three theorems about intersecting chords, intersecting secants, and the square on a tangent. The first theorem deals with chords that intersect within the circle.

In this situation, each chord cuts the other into two sub-intervals called intercepts. It is an amazing consequence of similar triangles that, in this situation, the products of the intercepts on each chord are equal. When two chords intersect within a circle, the products of the intercepts are equal. Join AP and BQ. The very last step is particularly interesting. It converts the equality of two ratios of lengths to the equality of two products of lengths. This is a common procedure when working with similarity. Many problems involving similarity can be handled using the sine rule. The exercise below gives an alternative proof of the intersecting chord theorem using the sine rule to deal directly with the ratio of two sides of the triangles. When you want to find the sine, cosine, or tangent of an angle, you use the following formulas: Draw a diagram after reading the problem.

Sometimes the problem will be provided without an image and you will have to diagram it yourself to visualize the proof. Once you have a rough sketch that matches the givens in a problem, you might need to re-draw the diagram so that you can read everything clearly and the angles are approximately correct. Make sure to label everything very clearly based on the information provided. The clearer your diagram, the easier it will be to think through the proof. Make some observations about your diagram. Label right angles and equal lengths. If lines are parallel to each other, mark that down as well.

If the problem does not explicitly state two lines are equal, can you prove that they are? Make sure you can prove all of your assumptions. Write down the relationships between various lines and angles that you can conclude based on your diagram and assumptions. Write down the givens in the problem. In any geometric proof, there is some information that is given by the problem. Writing them down first can help you think through the process needed for the proof. Work the proof backwards. When you are proving something in geometry, you are given some statements about the shapes and angles, then asked to prove why these statements are true.

Sometimes the easiest way to do this is to start with the end of the problem. How does the problem come to that conclusion? Are there a few obvious steps that must be proved to make this work? Make a 2-column grid labeled with statements and reasons. In order to make a solid proof, you have to make a statement and then give the geometric reason that proves the truth of that statement. Under the reason, you will write the proof for this. If it is given, simply write given, otherwise, write the theorem that proves it.

Determine which theorems apply to your proof. There are many individual theorems in geometry that can be used for your proof. There are many properties of triangles, intersecting and parallel lines, and circles that are the basis for these theorems. Determine what geometric shapes you are working with and find the ones that apply to your proof.

Reference previous proofs to see if there are similarities. There are too many theorems to list, but here are a few of the most important ones for triangles: Make sure your steps flow in a logical fashion. Write down a quick sketch of your proof outline. Write down the reasons for each step. Add the given statements where they belong, not just all at once in the beginning. Re-order the steps if necessary.

The more proofs you do, the easier it will be to order the steps properly. Write down the conclusion as the last line. The final step should complete your proof, but it still needs a reason to justify it.

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## Calculus I

When you have finished the proof, look it over and make sure there are no gaps in your reasoning. Once you have determined that the proof is sound, write QED at the bottom right corner to signify it is complete. What is a good way to learn the concept of angles, tangents and transverses? It is a math so it requires practice. Try at least one hour a day to practice it. First, try to learn all concepts and then directly jump on solving equations. You would not immediately be perfect as it takes time to learn math.

Get phone numbers and emails of several people in your geometry class so they can help you while you're studying at home.

### Why Is This Useful?

Take a class in the summer beforehand so you don't have to work hard during the school year. In other words, we will be finding the largest and smallest values that a function will have. The Shape of a Graph, Part I — In this section we will discuss what the first derivative of a function can tell us about the graph of a function. The first derivative will allow us to identify the relative or local minimum and maximum values of a function and where a function will be increasing and decreasing.

We will also give the First Derivative test which will allow us to classify critical points as relative minimums, relative maximums or neither a minimum or a maximum. The Shape of a Graph, Part II — In this section we will discuss what the second derivative of a function can tell us about the graph of a function. The second derivative will allow us to determine where the graph of a function is concave up and concave down.

The second derivative will also allow us to identify any inflection points i. We will also give the Second Derivative Test that will give an alternative method for identifying some critical points but not all as relative minimums or relative maximums. With the Mean Value Theorem we will prove a couple of very nice facts, one of which will be very useful in the next chapter.

We will discuss several methods for determining the absolute minimum or maximum of the function. Examples in this section tend to center around geometric objects such as squares, boxes, cylinders, etc. More Optimization Problems — In this section we will continue working optimization problems.

The examples in this section tend to be a little more involved and will often involve situations that will be more easily described with a sketch as opposed to the 'simple' geometric objects we looked at in the previous section. Linear Approximations — In this section we discuss using the derivative to compute a linear approximation to a function. We can use the linear approximation to a function to approximate values of the function at certain points.

While it might not seem like a useful thing to do with when we have the function there really are reasons that one might want to do this. We give two ways this can be useful in the examples. Differentials — In this section we will compute the differential for a function. We will give an application of differentials in this section. However, one of the more important uses of differentials will come in the next chapter and unfortunately we will not be able to discuss it until then. Newton's Method is an application of derivatives will allow us to approximate solutions to an equation.

There are many equations that cannot be solved directly and with this method we can get approximations to the solutions to many of those equations. Business Applications — In this section we will give a cursory discussion of some basic applications of derivatives to the business field. Note that this section is only intended to introduce these concepts and not teach you everything about them. Indefinite Integrals — In this section we will start off the chapter with the definition and properties of indefinite integrals.

We will not be computing many indefinite integrals in this section. This section is devoted to simply defining what an indefinite integral is and to give many of the properties of the indefinite integral. Actually computing indefinite integrals will start in the next section.

Computing Indefinite Integrals — In this section we will compute some indefinite integrals. The integrals in this section will tend to be those that do not require a lot of manipulation of the function we are integrating in order to actually compute the integral. As we will see starting in the next section many integrals do require some manipulation of the function before we can actually do the integral. We will also take a quick look at an application of indefinite integrals. Substitution Rule for Indefinite Integrals — In this section we will start using one of the more common and useful integration techniques — The Substitution Rule.

With the substitution rule we will be able integrate a wider variety of functions. The integrals in this section will all require some manipulation of the function prior to integrating unlike most of the integrals from the previous section where all we really needed were the basic integration formulas. More Substitution Rule — In this section we will continue to look at the substitution rule. The problems in this section will tend to be a little more involved than those in the previous section.

Area Problem — In this section we start off with the motivation for definite integrals and give one of the interpretations of definite integrals.

## High School Geometry | Khan Academy

As we will see in the next section this problem will lead us to the definition of the definite integral and will be one of the main interpretations of the definite integral that we'll be looking at in this material. Definition of the Definite Integral — In this section we will formally define the definite integral, give many of its properties and discuss a couple of interpretations of the definite integral. We will also look at the first part of the Fundamental Theorem of Calculus which shows the very close relationship between derivatives and integrals.

Computing Definite Integrals — In this section we will take a look at the second part of the Fundamental Theorem of Calculus.

This will show us how we compute definite integrals without using the often very unpleasant definition. The examples in this section can all be done with a basic knowledge of indefinite integrals and will not require the use of the substitution rule. Included in the examples in this section are computing definite integrals of piecewise and absolute value functions. Substitution Rule for Definite Integrals — In this section we will revisit the substitution rule as it applies to definite integrals. Geometry Quick Review: Triangles - Key Theorems and Proofs (Quick Review Notes) Geometry Quick Review: Triangles - Key Theorems and Proofs (Quick Review Notes) Geometry Quick Review: Triangles - Key Theorems and Proofs (Quick Review Notes) Geometry Quick Review: Triangles - Key Theorems and Proofs (Quick Review Notes) Geometry Quick Review: Triangles - Key Theorems and Proofs (Quick Review Notes)

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