In this page, we will will learn about the Double Angle Formulae used in Trigonometry. It is actually quite rare that exam questions are solely about these identities, but it is essential that you can use and manipulate them confidently because they are used in so many different parts of the course (so they do come up a lot!). You will learn what they are and how to use them.
On this page, you should learn about the double angle identities for sine and cosine
\(\large\sin2\theta \equiv 2\sin \theta \cos \theta \)
\(\large{\cos2\theta \equiv \cos^2\theta -\sin^2\theta\\ \cos2\theta \equiv 2\cos^2\theta -1\\ \cos2\theta \equiv 1 -2\sin^2\theta}\)
\(\large \tan2θ≡\frac{2\tanθ}{1-\tan^2θ}\)
Print from here
How much of Double Angle Formulae HL have you understood?