For the past month, other than being busy with the June Intensive Revision Programmes, I was also packed with Mid Year Exam Papers Review for my group of students.

I am pleased to announce that almost all **see an improvement** in their Mid Year Results (One boy has his grades jump from F9 to A2! Congrats, B!)

I was reviewing the school a-Math exam papers with one of my students during my A-Math Ultimate Leap Programme when she posted me a good question.

Let's take a look at her Trigonometry question first:

This was her solution for the question:

There is one misconception in her thoughts, can you spot it?

**sin 3x is not equal to sin x + sin 2x! **

In fact, the correct formula to choose for this question is

**sin 3x = sin (2x + x) **

** = sin2xcosx + cos2xsinx**

Her question was: how do you know which formula to use?

**TIP:** When the angle involved is **ODD**, we usually use **addition formula** (just like in this question).

When angle is **EVEN,** **double angle formula** is perhaps the correct one to choose.

Are you confused just like my student, which formula should be use? Leave me a comment.

I hope this post has made your understanding clearer :-)

Below is a list of useful posts on A-Math Trigonometry:

- How To Prove Trigonometry Identity
- A-Math Trigo Graphs
- More A-Math Trigo Graphs
- Drawing of Trigo Graphs

**PS:** I will be re-running A-Math Trigonometry Mastery Workshop in July as some students miss out the one in June due to other commitments. Registration is open! Click here for more details