You are Here: Home >< Maths

# Roots and trig watch

1. Not sure how to approach C. Tried saying that cos(3x) is root 2 but that's not valid; so there can't be 3 roots, I know that much from that. Maybe expressing root 2 in terms of cos(15) or sin(15)? But I'm unsure where to proceed from that.

a) Got

b) I said

and after using compound angle formulae on the right side, I got

so is a root.

Other two roots are; .
2. (Original post by RDKGames)

Not sure how to approach C. Tried saying that cos(3x) is root 2 but that's not valid; so there can't be 3 roots, I know that much from that. Maybe expressing root 2 in terms of cos(15) or sin(15)? But I'm unsure where to proceed from that.

a) Got

b) I said

and after using compound angle formulae on the right side, I got

so is a root.

Other two roots are; .
start by letting 2x=y and then dividing through by 2
3. (Original post by RDKGames)
...

Note that so the above is...
4. (Original post by ValerieKR)
start by letting 2x=y and then dividing through by 2
Okay that one caught me out completely. Didn't even notice the coefficients weren't the same :/
Thanks, I'll give this a go now.

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: September 8, 2016
Today on TSR

### Three reasons you may feel demotivated right now

...and how to stay positive

### Can I get A*s if I start revising now?

Discussions on TSR

• Latest
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
Discussions on TSR

• Latest

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