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C3 Trig: Non-real errors when finding the roots of quadratic trig functions

fffffffffff.JPG
I only lost one mark from the above question, and that was because I missed out two angles of the possible four. The mark scheme for the question is here:

https://imgur.com/a/k0Eidvo

When you get to the stage of finding the roots, the positive root,
((-3+√17)/4) gives you 16.3, which allows you to find the angles 16.3 and 163.7. The negative root ((-3-√17)/4) is a non-real error, but the mark scheme seems to treat this as equalling 0, which is where it finds 0 and 180. Why is this the case? I assumed that as it was non-real, it was to be discarded. 0 itself can be a root / achievable answer, so what's happening here? Thanks for any input!
Original post by Dggj_19
fffffffffff.JPG
I only lost one mark from the above question, and that was because I missed out two angles of the possible four. The mark scheme for the question is here:

https://imgur.com/a/k0Eidvo

When you get to the stage of finding the roots, the positive root,
((-3+√17)/4) gives you 16.3, which allows you to find the angles 16.3 and 163.7. The negative root ((-3-√17)/4) is a non-real error, but the mark scheme seems to treat this as equalling 0, which is where it finds 0 and 180. Why is this the case? I assumed that as it was non-real, it was to be discarded. 0 itself can be a root / achievable answer, so what's happening here? Thanks for any input!

The mark scheme gets 0 and 180 from tanx(cos2x - 3sinx)=0 - either cos2x-3sinx=0 (which is where you got your two solutions) or tanx=0.
Original post by Dggj_19

I only lost one mark from the above question, and that was because I missed out two angles of the possible four. The mark scheme for the question is here:

https://imgur.com/a/k0Eidvo

When you get to the stage of finding the roots, the positive root,
((-3+√17)/4) gives you 16.3, which allows you to find the angles 16.3 and 163.7. The negative root ((-3-√17)/4) is a non-real error, but the mark scheme seems to treat this as equalling 0, which is where it finds 0 and 180. Why is this the case? I assumed that as it was non-real, it was to be discarded. 0 itself can be a root / achievable answer, so what's happening here? Thanks for any input!


You're thinking about it in the wrong way.

The 0 and 180 solutions come from having tan⁡x=0\tan x = 0
Original post by Dggj_19
fffffffffff.JPG
I only lost one mark from the above question, and that was because I missed out two angles of the possible four. The mark scheme for the question is here:

https://imgur.com/a/k0Eidvo

When you get to the stage of finding the roots, the positive root,
((-3+√17)/4) gives you 16.3, which allows you to find the angles 16.3 and 163.7. The negative root ((-3-√17)/4) is a non-real error, but the mark scheme seems to treat this as equalling 0, which is where it finds 0 and 180. Why is this the case? I assumed that as it was non-real, it was to be discarded. 0 itself can be a root / achievable answer, so what's happening here? Thanks for any input!

They are the solutions of the other factor tan x = 0
Reply 4
Avatar for Dggj_19
Dggj_19
OP
11
Original post by I hate maths
The mark scheme gets 0 and 180 from tanx(cos2x - 3sinx)=0 - either cos2x-3sinx=0 (which is where you got your two solutions) or tanx=0.


Got it, thank you!
Original post by Dggj_19
Got it, thank you!


When I posted there were no other replies ... why are other posts there before mine now? Do some posters have time machines?
Original post by Dggj_19
Got it, thank you!

No problem.. I'll give you a pro tip: read the examiner reports on question you don't understand why you're wrong. It's helped me immensely before.

Reply 7
Avatar for Notnek
Notnek
21
Original post by Muttley79
When I posted there were no other replies ... why are other posts there before mine now? Do some posters have time machines?

I wonder if two threads were merged? But that wouldn’t explain why the time of your post is earlier than the ones above it. Weird.

EDIT: now the times are different...
(edited 5 years ago)
Reply 8
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Dggj_19
OP
11
Original post by Muttley79
When I posted there were no other replies ... why are other posts there before mine now? Do some posters have time machines?


This site is p slow to update. It may well have been a case that you opened the actual link before there were any answers, but by the time you got round to posting your answer, there were already submitted answers.
Original post by Dggj_19
This site is p slow to update. It may well have been a case that you opened the actual link before there were any answers, but by the time you got round to posting your answer, there were already submitted answers.

No I actually looked at the thread after I posted - no other response was there.
you may have divided out tanx from both sides.

in fact you should factorise it out then you keep the extra solution values.

tanx = 0 =====> x = 0º, 180º
Reply 11
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Dggj_19
OP
11
Original post by I hate maths
No problem.. I'll give you a pro tip: read the examiner reports on question you don't understand why you're wrong. It's helped me immensely before.



Oh damn, this looks incredibly useful. Where exactly can you find these? From a quick glance they don't seem to be included on the PMT Past Papers page, which is where I source all my questions.
Original post by Dggj_19
Oh damn, this looks incredibly useful. Where exactly can you find these? From a quick glance they don't seem to be included on the PMT Past Papers page, which is where I source all my questions.


You can find them on the official Edexcel website. https://qualifications.pearson.com/en/support/support-topics/exams/past-papers.html/student
Reply 13
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Dggj_19
OP
11
Original post by I hate maths

Brilliant, I'll definitely be using these. Thanks again
Original post by Dggj_19
Brilliant, I'll definitely be using these. Thanks again

Hasn't your teacher mentioned these? I think they are often more useful than a mark scheme.
Reply 15
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Dggj_19
OP
11
Original post by Muttley79
Hasn't your teacher mentioned these? I think they are often more useful than a mark scheme.


I'm a private candidate, teaching myself for this year. I would be taking a third year in college, but my college stopped offering A-Level at the end of last academic year, which left me high and dry as far as resits were concerned. It may have been mentioned, but it's been a while xD

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