# O Level A-Math Trigonometry Basics : How Do We Know Which Formula To Use?

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:

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

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### Ai Ling Ong

Hi, I'm Ai Ling Ong. 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... If you have found this blog post useful, please share it with your friends. I will really appreciate it! :)

### 18 Responses to O Level A-Math Trigonometry Basics : How Do We Know Which Formula To Use?

1. I don't like math but as i realized, math is very useful in life! too bad that children don't realize this sooner

2. desiree says:

im still confuse...

What are you confused about?

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

3. real says:

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

4. Nguy?n ??c Tùng says:

It is very Useful for me.

5. sm says:

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

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

6. meera says:

modulus graphs - method is so convenient.
thank u very much

7. ariane says:

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

8. gautam says:

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

9. Pho Tharr says:

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

10. Aayush says:

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

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

12. Darren says:

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

13. Darren says:

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