How can I remember trig identities?

Trigonometric identities are sometimes tricky, as they are very hard to distinguish one from another. The best way of doing it is obviously by practising, but one thing I always find helpful is to give an example. In the case of cos(a+b), we expand it as cos(a) * cos(b) - sin(a) * sin(b). How can I check that I am sure? Well, if b = 0, then the right hand side term becomes cos(a)*1-sin(a)*0 = cos(a) = cos(a+0). Checked. Is cos(-a-b) = cos(a+b)? Yes, it is. cos(-a)*cos(-b) = cos(a) * cos(b), as cos is an even function and sin(-a)*sin(-b) = -sin(a)*(-sin(b)) = sin(a)*sin(b), as sin is odd. Hence the identity is unchanged.

So, we have taken to important properties of trigonometric functions which are still available on our guessed identity, thus we strongly believe is true, which it is.

Marius S. A Level Maths tutor, GCSE Maths tutor

1 year ago

Answered by Marius, who tutored A Level Maths with MyTutor

Still stuck? Get one-to-one help from a personally interviewed subject specialist


£20 /hr

Michael W.

Degree: Aerospace Engineering (Masters) - Bristol University

Subjects offered: Maths, Science+ 2 more


“I am a fourth year aerospace engineering student at the University of Bristol. I have a passion for science and mathematics as it can be applied to almost everything and enables us to solve challenging problems! I hope to share my kno...”

MyTutor guarantee

£20 /hr

Will W.

Degree: Mathematics (Bachelors) - Durham University

Subjects offered: Maths, Physics+ 2 more

Further Mathematics

“About Me: I am a Maths and Physics student at Durham University, who has always enjoyed understanding complex concepts in these subjects. I also have a passion for sharing this enjoyment. At my previous school, I ran a Maths Club. Thi...”

£20 /hr

Jack G.

Degree: Politics, Philosophy, and Economics (Bachelors) - Warwick University

Subjects offered: Maths, Religious Studies+ 2 more

Religious Studies
.TSA. Oxford.

“Hello, I'm Jack and am studying politics, philosophy, and economics at Warwick.  Having recently been through the subjects I'm looking to tutor I feel like I know first hand what difficulties you'll be facing. I always found I wanted ...”

MyTutor guarantee

About the author

£20 /hr

Marius S.

Degree: Applied Mathematics (Bachelors) - Edinburgh University

Subjects offered: Maths


“Top tutor from the renowned Russell university group, ready to help you improve your grades.”

MyTutor guarantee

You may also like...

Other A Level Maths questions

The quadratic equation 2x^2 + 8x + 1 = 0 has roots a and b. Write down the value of a + b, a*b and a^2 + b^2.

What is the tangent line to the curve y = x^3+4x+5 at the point where x = 2?

How do you integrate xcos(x)?

Differentiate x^2

View A Level Maths tutors


We use cookies to improve our service. By continuing to use this website, we'll assume that you're OK with this. Dismiss