# Double angle and half angle identities

## Double Angle Identities Special cases of the sum and difference formulas for sine and cosine yields what is known as the double?angle identities and the half?angle identities. First, u.

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In mathematics , trigonometric identities are equalities that involve trigonometric functions and are true for every value of the occurring variables where both sides of the equality are defined. Geometrically, these are identities involving certain functions of one or more angles. They are distinct from triangle identities , which are identities potentially involving angles but also involving side lengths or other lengths of a triangle. These identities are useful whenever expressions involving trigonometric functions need to be simplified. An important application is the integration of non-trigonometric functions: a common technique involves first using the substitution rule with a trigonometric function , and then simplifying the resulting integral with a trigonometric identity.

In this section, you will learn formulas that establish a relationship between the basic trigonometric values sin, cos, tan for a particular angle and the trigonometric values for an angle that is either double- or half- of the first angle. These relationships can be very useful in proofs and also in problem solving because they can often be used to simplify an equation. Recall the three summation formulas:. It is also important to note that the following relationships are NOT true:. There is one important thing to note about these two equations. For example, in the quadratic equation, there are two answers - one for the positive version and one for the negative version of the radical. However, in this case, only one answer either posititive or negative should be selected.

Playing trial and error with charts or a calculator to find the answer would range from a drawn-out nightmare to totally impossible. Fortunately, the double-angle identities are here to help. There are three double-angle identities, one each for the sine, cosine and tangent functions. But the sine and cosine identities can be written in multiple ways. Here are the two ways of writing the double-angle identity for the sine function:. Imagine that you're faced with a right triangle where you know the length of its sides, but not the measure of its angles.

Following table gives the double angle identities which can be used while solving the equations. You can also have sin 2theta, cos 2theta expressed in terms of tan theta as under.
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You have seen quite a few trigonometric identities in the past few pages. It is convenient to have a summary of them for reference. The more important identities. But these you should. Defining relations for tangent, cotangent, secant, and cosecant in terms of sine and cosine. The Pythagorean formula for sines and cosines. This is probably the most important trig identity.

In trigonometry , tangent half-angle formulas relate the tangent of half of an angle to trigonometric functions of the entire angle. Among these are the following. From these one can derive identities expressing the sine, cosine, and tangent as functions of tangents of half-angles:. Also, using the angle addition and subtraction formulae for both the sine and cosine one obtains:. According to similar triangles ,. In various applications of trigonometry , it is useful to rewrite the trigonometric functions such as sine and cosine in terms of rational functions of a new variable t.

## Trigonometric Double-Angle and Half-Angle Formulas

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