# C3 trig helpWatch

Announcements
#1
I've tried this question twice and it should be fairly easy but I keep getting it wrong :/

cos2x = 1 - cosx

I originally put all the cosx on one side then factorised by taking out cos. that didn't work so I replaced cos2x with 2cos^2x-1 to form a quadratic which also didn't end in the correct answer :/

Posted from TSR Mobile
0
5 years ago
#2
Cos2x=1-cosx
Let x=45
Cos90=1-cos45
0=1-1/√2
LHS≠RHS
So cos2x≠1-cosx
0
5 years ago
#3
(Original post by kendellex)
I've tried this question twice and it should be fairly easy but I keep getting it wrong :/

cos2x = 1 - cosx

I originally put all the cosx on one side then factorised by taking out cos. that didn't work so I replaced cos2x with 2cos^2x-1 to form a quadratic which also didn't end in the correct answer :/

Posted from TSR Mobile
0
#4
(Original post by m4ths/maths247)
I thought it was 2y^2 + y - 2 = 0 were y = cosx. It doesn't produce the correct answer though so must be wrong

Posted from TSR Mobile
0
5 years ago
#5
(Original post by krisshP)
Cos2x=1-cosx
Let x=45
Cos90=1-cos45
0=1-1/√2
LHS≠RHS
So cos2x≠1-cosx
I'm not sure what you are trying to do here?
0
5 years ago
#6
(Original post by kendellex)
I thought it was 2y^2 + y - 2 = 0 were y = cosx. It doesn't produce the correct answer though so must be wrong

Posted from TSR Mobile
Have you solved for x and not cosx?
There are certainly 2 solutions between 0 and 2pi
1
5 years ago
#7
2 cos^2(x) - 1 =1 - cosx

2 cos^2(x) + cosx - 2 =0

cosx= (1
= (1
+/−3)/4
cosx= 1 OR -1/2

0
5 years ago
#8
(Original post by tsumannai)
2 cos^2(x) - 1 =1 - cosx

2 cos^2(x) + cosx - 2 =0

cosx= (1
= (1
+/−3)/4
cosx= 1 OR -1/2

The board etiquette is such that full solutions are not posted.
This has not answered the question anyhow....
0
5 years ago
#9
(Original post by m4ths/maths247)
I'm not sure what you are trying to do here?
Oh no, sorry, I thought that we were supposed to prove that cos2x is identically equal to 1-cosx, now I realise we're we're solving cos2x=1-cosx after looking at post 7. After all there are many trig questions based on proving.
0
X

new posts
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

### University open days

• Brunel University London
Wed, 22 May '19
• University of Roehampton
Wed, 22 May '19
• Sheffield Hallam University
Wed, 22 May '19

### Poll

Join the discussion

#### How did your Edexcel GCSE Maths Paper 1 go?

Loved the paper - feeling positive (39)
24.38%
The paper was reasonable (69)
43.13%
Not feeling great about that exam (27)
16.88%
It was TERRIBLE (25)
15.63%