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

### Related Posts:

Hi, I'm Ai Ling. I enjoy coaching students who have challenges with understanding and scoring in 'O' Level A-Maths and E-Maths.
I develop Math strategies, sometimes ridiculous ideas to help students in understanding abstract concepts the fast and memorable way.
I write this blog to share with you the stuff I teach in my class, the common mistakes my students made, the 'way' to think, analyze...
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I don’t like math but as i realized, math is very useful in life! too bad that children don’t realize this sooner

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im still confuse…

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alwaysLovely Reply:

August 11th, 2009 at 11:12 am

What are you confused about?

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arun Reply:

February 6th, 2010 at 1:01 pm

i bunked few of ma classes so got doubt in allied angles

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Wow this was really informative! I didnt know about that haha

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It is very Useful for me.

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thanks this came out as a part of a question in my prelim now i noe how to do it =D

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alwaysLovely Reply:

September 11th, 2009 at 7:12 am

I’m glad you know how to do the question now!

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modulus graphs – method is so convenient.

thank u very much

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Hi, i am so happy to know you.I want to ask if i can also get information about A-LEVEL maths

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i am unable to understand this :(… plz explain in more detail or do it frm the basic plz….

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It is useful for me! Thanks! Please send me another amazing things.

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why do i need a o levl maths for admission in a international university

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Hi!! yes.. i was a bit confuse ..through your explaination, it beame clear now

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I just don’t know about prove the identity,i don’t understand how could it prove to be useful to our daily life..

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i don’t know a bit about rate of change, could someone explain it via the same website??

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6&8 of june em gona attempt my cambridge finel exam and stii know i don’t know about division

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I still don’t get which formula you use if you want to find the length or an angle. Please help.

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