Lesson Explainer: Double-Angle and Half-Angle Identities Mathematics • 10th Grade

In this explainer, we will learn how to use the double-angle and half-angle identities to evaluate trigonometric values.

The double-angle identities give and in terms of and . They are a special case of the sum identities, and , namely when . The double-angle identities can be simply derived from them as shown in the following:

It is worth pausing here for a moment and reflecting on the respective signs of , , and as functions of . As shown in the following figure, if we let be in a half-quadrant of the unit circle, then is in a whole quadrant.

For instance, when (or ), we have (or ). In this case, , , , , , and are all nonnegative, and is not defined for .

Let us check that this is consistent with the double-angle identities. The first identity is . Since , we have , and thus, , which corresponds indeed to being nonnegative.

We derive from the second identity, , that is nonnegative if and have the same sign or one of them is zero. This is the case for .

Finally, since when , we have , which leads to , and thus, .

We can apply similar considerations for all 8 half-quadrants of the unit circle (for the 4 half-quadrants at the bottom of the circle, the values of are then greater than or rad) and show that the double-angle identities hold for any values of .

We can use these two to derive a formula for too. In this case, purely in terms of , giving us and

Let us look with our first example at how to use these double-angle identities to simplify trigonometric expressions.

Let us take a note of the two identities that we derived from in the previous example.

Now, we will see an example of how we can simplify a trigonometric expression by using the double-angle identity in the opposite direction.

Let us now evaluate a trigonometric expression using the double-angle identity.

So far, we have used the double-angle identities. The half-angle identities can be derived from them simply by realizing that the difference between considering one angle and its double and considering an angle and its half is just a matter of perspective.

Using the identity that we derived from the double-angle identity and substituting in and , we get

Starting from the other identity derived from the double-angle identity (i.e., ), we find that

Finally, to find , we use the expressions we just established for and by writing

With the three half-angle identities we have derived above, we see that the absolute values of the three trigonometric functions of half an angle depend only on the value of the cosine of this angle. However, the “” shows that their signs are not determined. This means that we will be able to write the correct sign in the identity only if we know in which quadrant the half angle is located.

This comes from the fact that an angle is not fully determined by its cosine value. Take for instance , , , and . While we have , we see that and , as shown in the following diagram.

Let us now see how the tangent half-angle identity can be rewritten so that the sign is determined.

Starting with we multiply the numerator and denominator of the fraction on the right-hand side by , which gives

The numerator can be rewritten as using the double-angle identities (with and ) and the denominator as using the identity (with and ) derived from the double-angle identity .

Hence, we have

Starting again from we now multiply the numerator and denominator of the fraction on the right-hand side by . We get

Using here the identity , we can rewrite as and, as before, as . Hence, we find that

It is worth noting that rearranging by multiplying the right-hand side of the equation by either or leads to and . Studying the respective signs of and as shown in the following table, we conclude that and always have the same sign. This allows us to establish the identities and .

Let us recap the half-angles identities that we have just derived.

Let us look at how to use the half-angle identities in the following example.

Finally, let us see with our last example how the double-angle and half-angle identities can be used to find the exact value of some trigonometric functions.

Let us now summarize what we have learned in this explainer.