You are Here: Home >< Maths

# C3 Trig Question watch

1. I'm trying to solve a C3 trig question involving trig identities and whilst two of my solutions are correct, I'm missing a solution and the model answer completes the problem in a completely different way. Can anyone tell me where I'm going wrong? Thanks.

Spoiler:
Show

Spoiler:
Show

(and gets the same solutions as me as well as 90 degrees)
2. You can't cancel the cos in your 3rd line of working. That takes out a solution
3. (Original post by ryanroks1)
...
If I had the equation , do you agree that it would be folly of us to cancel an from both sides and get ? Surely you know that what you really need to do is which gets you two solutions.

The same applies with your trigonometric equation. Do not cancel the , you should bring it over to the other side and factorise it out whereby you can say that another set of solutions is given by .
4. (Original post by Zacken)
If I had the equation , do you agree that it would be folly of us to cancel an from both sides and get ? Surely you know that what you really need to do is which gets you two solutions.

The same applies with your trigonometric equation. Do not cancel the , you should bring it over to the other side and factorise it out whereby you can say that another set of solutions is given by .
Great explanation as usual!

Just a side note - would you say we cannot cancel the cos because it's actually equal to 0 in one solution, and so dividing by 0 is not possible?

Or is what I'm saying rubbish and only applicable to this example?
5. (Original post by Student403)
Or is what I'm saying rubbish and only applicable to this example?
Nope, that's precisely why.
6. (Original post by Zacken)
Nope, that's precisely why.
Wow that's really good to know - thank you
7. (Original post by Student403)
x
(Original post by Zacken)
x.
Thank you, seems obvious now!
8. (Original post by ryanroks1)
Thank you, seems obvious now!
Nice going
9. (Original post by Student403)
You can't cancel the cos in your 3rd line of working. That takes out a solution
Why are you giving advice when its incorrect?
10. (Original post by GeologyMaths)
Why are you giving advice when its incorrect?
Why are you acting condescending when you haven't yet stated what the "correct" advice would be?
11. (Original post by Student403)
Why are you acting condescending when you haven't yet stated what the "correct" advice would be?
correct advise is to minus cos then factorise
12. (Original post by GeologyMaths)
correct advise is to minus cos then factorise
OP asked "Where am I going wrong?", which I clearly answered in my post

### Related university courses

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: March 8, 2016
Today on TSR

### He broke up with me because of long distance

Now I'm moving to his city

### University open days

1. Norwich University of the Arts
Thu, 19 Jul '18
2. University of Sunderland
Thu, 19 Jul '18
3. Plymouth College of Art
Thu, 19 Jul '18
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

## Groups associated with this forum:

View associated groups

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE