| Introduction to trigonometryAs you see, the word itself refers to three angles - a reference to triangles. Trigonometry is primarily a branch of mathematics that deals with triangles, mostly right triangles. In particular the ratios and relationships between the triangle's sides and angles. It has two main ways of being used: 1. In geometryIn its geometry application, it is mainly used to solve triangles, usually right triangles. That is, given some angles and side lengths, we can find some or all the others. For example, in the figure below, knowing the height of the tree and the angle made when we look up at its top, we can calculate how far away it is (CB). (Using our full toolbox, we can actually calculate all three sides and all three angles of the right triangle ABC). 2. AnalyticallyIn a more advanced use, the trigonometric ratios such as as Sine and Tangent, are used as functions in equations and are manipulated using algebra. In this way, it has many engineering applications such as electronic circuits and mechanical engineering. In this analytical application, it deals with angles drawn on a coordinate plane, and can be used to analyze things like motion and waves. Chapter-1Angles in the Quadrants( Some basic Concepts)In trigonometry, an angle is drawn in what is called the "standard position". The vertex of the angle is on the origin, and one side of the angle is fixed and drawn along the positive x-axis.Names of the partsThe side that is fixed along the positive x axis (BC) is called the initial side. To make the angle, imagine of a copy of this side being rotated about the origin to create the second side, called the terminal side. The amount we rotate it is called the measure of the angle and is measured in degrees or radians. This measure can be written in a short form: mABC = 54° which is spoken as "the measure of angle ABC is 54 degrees". If it is not ambiguous, we may use just a single letter to denote an angle. In the figure above, we could refer to the angle as ABC or just angle B. In trigonometry, you will often see Greek letters used to name angles. For example the letter θ (theta), but on this site we always use ordinary letters like A,B,C. The measure can be positive or negativeBy convention, angles that go counterclockwise from the initial side are positive and those that go clockwise are negative. In the figure above, click on 'reset'. The angle shown goes counterclockwise and so is positive. Drag A down across the x-axis and see that angles going clockwise from the initial side are negative. See Trig functions of large and negative angles The measure can exceed 360°In the figure above click 'reset' and drag the point A around counterclockwise. Once you have made a full circle (360°) keep going and you will see that the angle is greater than 360°. In fact you can go around as many times as you like. The same thing happens when you go clockwise. The negative angle just keeps on increasing. Coterminal anglesIf you have one angle of say 30°, another of 390°, the two terminal sides will be in the same place (390 = 360+30). These two angles would then be called coterminal angles. They would be in the same place on the plane but have different measures (30° and 390°). Degrees and radiansThe measure of an angles can be expressed in degrees or radians, but in trigonometry radians are the most common. See Radians and Degrees. Recall than there are 2π radians in a full circle of 360°, so 1 radian is approximately 57°. In the figure above, click on "radians" to change units.
| Standard position of an angle
In trigonometry an angle is usually drawn in what is called the "standard position" as shown below.
In this position, the vertex of the angle (B) is on the origin of the x and y axis. One side of the angle is always fixed along the positive x-axis - that is, going to the right along the axis in the 3 o'clock direction (line BC). This is called the initial side of the angle. The...
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