712 views

### What is the best way to prove trig identities?

In my experience the sure fire way to prove trig identities is by doing the following:

1) Assess the question e.g. is it obvious you're going to need a double angle or addition formula, can you see where cos^2(x) + sin^2(x) = 1 would be needed etc.

2) Write down all the formulae you might need (it's also worth noting that two identities are easily proved by dividing cos^2(x) + sin^2(x) = 1 by cos^2(x) and another by sin^2(x) namely 1+tan^2(x) = sec^2(x) and cot^2(x) + 1 = csc^2(x).

3) Work from the more complicated side and reduce it to the simpler side. To prove that A = B is the same as proving B = A so it doesn't matter which way you start.

4) Some general tips if it's especially difficult. Try maybe factoring and seeing if a trig identity appears, try multiplying by 1 or adding 0 in "clever" ways. e.g. Maybe multiply by (sinx+1/sinx+1) then you haven't changed anything but it might be in a more useful form. Maybe also write simpler expressions as something else in case that's useful, e.g. instead of tanx write sinx/cosx. Finally if you have a fraction it might be worth multiplying both sides by the denominator and see if it's in a nicer form.

A little example I made up:

Show
2sin^2(x) + 2cos^2(x) = (cos 2x)/(cos^2(x)) + sec^2(x)

1) I can see I'll likely need the double angle formula for cos 2x but bear in mind there are 3 of those. I also notice on the left hand side there's no division and sinx and cosx are most familiar so I'm going to say the right hand side is harder so I'll start work from  there.

2) I see I've got cos(2x) so I might need cos^2(x) -  sin^2(x), I can also see I've got sec^2(x) so I might need tan^2(x) +1 = sec^2(x)

Let's begin.

RHS(right hand side)

= cos(2x)/(cos^2(x)) + sec^2(x) (what we're given)

= (cos^2(x) - sin^2(x))/(cos^2(x))    + sec^2(x)     (expanding cos(2x) )

= 1 - sin^2(x)/cos^2(x)     + sec^2(x)         ( carrying out division)

= 1 - tan^2(x) + sec^2(x)             ( realising sinx/cosx = tan(x))

= 1 + 1                                 (using sec^2(x) - tan^2(x) = 1)

=2

But notice we can "cleverly" multiply by 1 (sin^2(x) + cos^2(x)) to get the desired result.

It's definitely worth noting that I went the long way round to try and exersize more techniques but the far better way to do this is to expand cos(2x) as 2cos^2(x) -1 because then you're immediately left with 2 after doing the division and cancelling the sec^2(x) which dramatically speeds up the process.

2 years ago

Answered by Expired account, who tutored A Level Maths with MyTutor

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

#### 349 SUBJECT SPECIALISTS

£26 /hr

Degree: Mathematical and Theoretical Physics (Masters) - Oxford, Merton College University

Subjects offered:Maths, Science+ 5 more

Maths
Science
Physics
Further Mathematics
Chemistry
.MAT.
-Personal Statements-

“Mathematics and Theoretical Physics, University of Oxford. I enjoy sharing my experience and enthusiasm in Maths with those who could do with some help”

£26 /hr

Degree: Mathematics (Masters) - Durham University

Subjects offered:Maths, Further Mathematics

Maths
Further Mathematics

“Hi! I’m Josh, a third year on a four-year Mathematics course. I got 100% in both my Maths IGCSE and A-Level, and did all 18 A-Level modules.”

£20 /hr

Degree: Economics (Bachelors) - Cambridge University

Subjects offered:Maths, Economics+ 1 more

Maths
Economics
-Personal Statements-

“Hey! My name is Mihir and I'm a Cambridge Economics student. I'm a friendly, hard-working tutor that will help students gain confidence in exams.”

MyTutor guarantee

£22 /hr

Degree: Mathematics (Bachelors) - Durham University

Subjects offered:Maths, Science+ 3 more

Maths
Science
Physics
Further Mathematics
Chemistry

“Hi, my name's Sam and I'm a mathematics (and physics) student at Durham. I know for some people the most important thing is just to get through their exams; I can andwill help you with this. I can't stress enough, however, how much e...”

### You may also like...

#### Posts by Expired account

How do you take the derivative of a^x ?

How does integration work?

What is the best way to prove trig identities?

#### Other A Level Maths questions

Use the substitution u = 6 - x^2 to find the value of the integral of (x^3)/(sqrt(6-x^2)) between the limits of x = 1 and x = 2 (AQA core 3 maths

Differentiate x^2

At x=3, is the polynomial y= (4/3)x^3 -6x^2 + 11 at a maxima or minima?

How do you find and solve a composite function?

We use cookies to improve your site experience. By continuing to use this website, we'll assume that you're OK with this.