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Mathematics LibreTexts

2: General Triangles

  • Page ID
    3217
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    In Section 1.3 we saw how to solve a right triangle: given two sides, or one side and one acute angle, we could find the remaining sides and angles. In each case we were actually given three pieces of information, since we already knew one angle was 90°. For a general triangle, which may or may not have a right angle, we will again need three pieces of information. The four cases are:

    • Case 1: One side and two angles
    • Case 2: Two sides and one opposite angle
    • Case 3: Two sides and the angle between them
    • Case 4: Three sides

    Note that if we were given all three angles we could not determine the sides uniquely; by similarity an infinite number of triangles have the same angles. In this chapter we will learn how to solve a general triangle in all four of the above cases. Though the methods described will work for right triangles, they are mostly used to solve oblique triangles, that is, triangles which do not have a right angle. There are two types of oblique triangles: an acute triangle has all acute angles, and an obtuse triangle has one obtuse angle. As we will see, Cases 1 and 2 can be solved using the law of sines, Case 3 can be solved using either the law of cosines or the law of tangents, and Case 4 can be solved using the law of cosines.

    • 2.1: The Law of Sines
      The Law of Sines states that the sides of a triangle are proportional to the sines of their opposite angles.
    • 2.2: The Law of Cosines
      We will now discuss how to solve a triangle where two sides and the angle between them are known. We will use the Law of Cosines to solve this problem.
    • 2.3: The Law of Tangents
      Law of Tangents is an alternative to the Law of Cosines for Case 3 scenarios (two sides and the included angle). Related to the Law of Tangents are Mollweide's equations.
    • 2.4: The Area of a Triangle
      In elementary geometry you learned that the area of a triangle is one-half the base times the height. We will now use that, combined with some trigonometry, to derive more formulas for the area when given various parts of the triangle.
    • 2.5: Circumscribed and Inscribed Circles
      Recall from the Law of Sines that any triangle has a common ratio of sides to sines of opposite angles. This common ratio has a geometric meaning: it is the diameter (i.e. twice the radius) of the unique circle in which the trianble can be inscribed, called the circumscribed circle of the triangle.
    • 2.E: General Triangles (Exercises)
      These are homework exercises to accompany Corral's "Elementary Trigonometry" Textmap. This is a text on elementary trigonometry, designed for students who have completed courses in high-school algebra and geometry. Though designed for college students, it could also be used in high schools. The traditional topics are covered, but a more geometrical approach is taken than usual. Also, some numerical methods (e.g. the secant method for solving trigonometric equations) are discussed.

    Thumbnails: If \(C \) is acute, then \(A \) and \(B \) are also acute. Since \(A \le C \), imagine that \(A \) is in standard position in the \(xy\)-coordinate plane and that we rotate the terminal side of \(A \) counterclockwise to the terminal side of the larger angle \(C \). If we pick points \((x_{1},y_{1}) \) and \((x_{2},y_{2}) \) on the terminal sides of \(A \) and \(C \), respectively, so that their distance to the origin is the same number \(r \), then we see from the picture that \(y_{1} \le y_{2} \).

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