You are Here: Home >< Maths

# C3 Problem, solving trig equations watch

1. Solve the equation for 0 < theta< 360

Sin 2theta-tan theta=0

is it best to use the double angle formula to expand first?
and would this give me

2sintheta cos theta- tan theta=0

where would I go from here because I have three signs
2. Change tan to sin/cos as well
3. As a general principle, I would change everything into signs and cosines of theta
4. multiply by cos, then divide by sine
5. i swear thats not C3!!!! Edexcel???
6. (Original post by 2strong)
i swear thats not C3!!!! Edexcel???
yes it is. it's pretty simple too.
7. I can't believe I missed changing the tan into sin/cos

but then again I'm sort of new to this. thanks
8. (Original post by milan5baros)
I can't believe I missed changing the tan into sin/cos

but then again I'm sort of new to this. thanks
when you get used to it, you'll do this sort of thing automatically, without having to write it out.
9. after some manipulation I get too sin theta(1-2sinsquared theta)

and from this I get the reference angle 45degrees, and then 135 degrees using CAST.

the book has about six answers, can anyone spot where I may have gone wrong here.

10. Does the question restrict theta to a certain range, because otherwise there are infinitely many values of theta which satisfy the equation
11. Yes, there is a range requirement. (0,360)

Although I have ended up with different solutions
12. Just saw it.

I got myself
13. x = theta

sin2x - tanx = 0
(2sinxcosx) - (sinx/cosx) = 0
2sinxcosx = sinx/cosx
2sinxcos^2x = sinx
2sinx(1 - sin^2x) = sinx

now just expand the brackets on t RHS and you'll be fine from there.

NB. I haven't done any maths since my about june so sorry if i'm wrong
14. Please don't put too much of a solution, hints are usually more helpfully, although what identities do we know involving
15. cos^2x = 1 - sin^2x

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: August 27, 2008
Today on TSR

### Fastest and slowest offer senders

Which unis still haven't sent offers?

### University open days

• University of Lincoln
Mini Open Day at the Brayford Campus Undergraduate
Wed, 19 Dec '18
• University of East Anglia
Fri, 4 Jan '19
• Bournemouth University
Wed, 9 Jan '19
Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams