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# C3 Trig Help Tweet

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1. C3 Trig Help

I've done part A, but a bit confused on part B

I know the Minimum is -5, but shouldn't this occur when sin(x+0.6435) = -1

On the mark scheme it occurs when sin(x+0.6435) = 3(pi)/2
2. Re: C3 Trig Help
(Original post by 0range)

I've done part A, but a bit confused on part B

I know the Minimum is -5, but shouldn't this occur when sin(x+0.6435) = -1

On the mark scheme it occurs when sin(x+0.6435) = 3(pi)/2
It should be

Can you give the link to the mark scheme.
3. Re: C3 Trig Help
(Original post by raheem94)
It should be

Can you give the link to the mark scheme.
It is my bad
It's downloaded so I'll try n upload it, but it's Soloman Paper C Q7
Attached Files
4. C3Cmarks.pdf (162.1 KB, 57 views)
5. Last edited by 0range; 13-06-2012 at 18:38.
6. Re: C3 Trig Help
(Original post by 0range)
It is my bad
It's downloaded so I'll try n upload it, but it's Soloman Paper C Q7

Do you get it?
7. Re: C3 Trig Help
(Original post by raheem94)

Do you get it?
Yh I did almost exactly that, but when I put Sin^-1(-1) I get -0.5(pi) :s
8. Re: C3 Trig Help
(Original post by 0range)
Yh I did almost exactly that, but when I put Sin^-1(-1) I get -0.5(pi) :s
We need the smallest positive value of .

We need the positive value hence we will use the other solution. The solution you used will give a negative value of

Does it makes sense?
9. Re: C3 Trig Help
(Original post by raheem94)
We need the smallest positive value of .

We need the positive value hence we will use the other solution. The solution you used will give a negative value of

Does it makes sense?

Yh it makes sense Thank you

Could you help me with one more thing?

For Rconversion I use Phythagerous for R and for alpha I use tan^-1(b/a)

Would there ever be a case when to work out alpha you would have to do tan-1(a/b)? or tan^-1(-b/a)
Sorry I know it's really obscure but I haven't come across anything like that in any of the past papers. Thanks again!
10. Re: C3 Trig Help
(Original post by 0range)
Yh it makes sense Thank you

Could you help me with one more thing?

For Rconversion I use Phythagerous for R and for alpha I use tan^-1(b/a)

Would there ever be a case when to work out alpha you would have to do tan-1(a/b)? or tan^-1(-b/a)
Sorry I know it's really obscure but I haven't come across anything like that in any of the past papers. Thanks again!
I don't generalize formulae, i do it by looking at the question.

I will approach these in the following way,

Now square both the above equations.

Now add

Now to find , do .

Do you get it?
11. Re: C3 Trig Help
(Original post by raheem94)
We need the smallest positive value of .

We need the positive value hence we will use the other solution. The solution you used will give a negative value of

Does it makes sense?
Hi sorry to intrude on the thread but I just had a quick question about these types of questions. When you want to express something in this form, once I find out the value of R I don't use tan to work out the alpha value. I use another method where I just compare what is similar in the orginal equation and what's not. Like, in the example that's on this thread, I would expand out R sin (x+ alpha) into R sin x cos alpha + R cos x sin alpha. Then with the first half, I would see that sin x is present in both equations so I would use R cos alpha = 4 and then solve it from there using cos-1 etc. I've not seen this quoted in the mark schemes but it always gives me the correct answer, would this method be alright to use Thanks so much!
12. Re: C3 Trig Help
(Original post by Bugsy)
Hi sorry to intrude on the thread but I just had a quick question about these types of questions. When you want to express something in this form, once I find out the value of R I don't use tan to work out the alpha value. I use another method where I just compare what is similar in the orginal equation and what's not. Like, in the example that's on this thread, I would expand out R sin (x+ alpha) into R sin x cos alpha + R cos x sin alpha. Then with the first half, I would see that sin x is present in both equations so I would use R cos alpha = 4 and then solve it from there using cos-1 etc. I've not seen this quoted in the mark schemes but it always gives me the correct answer, would this method be alright to use Thanks so much!
Both methods are fine
13. Re: C3 Trig Help
(Original post by raheem94)
I don't generalize formulae, i do it by looking at the question.

I will approach these in the following way,

Now square both the above equations.

Now add

Now to find , do .

Do you get it?
So imagine if it was 3sinx - 4cosx

then Rcosx = 3
Rsinx = -4

therefore it would be tan^-1(-4/3) ?
14. Re: C3 Trig Help
(Original post by raheem94)
Both methods are fine
Thank you!!
15. Re: C3 Trig Help
(Original post by 0range)
So imagine if it was 3sinx - 4cosx

then Rcosx = 3
Rsinx = -4

therefore it would be tan^-1(-4/3) ?
Express it as

For such questions . so we won't use your value.

Use my method.

Though if it is done by your method then also it can be made positive in the following way,
16. Re: C3 Trig Help
(Original post by raheem94)
Express it as

For such questions . so we won't use your value.

Use my method.

Though if it is done by your method then also it can be made positive in the following way,
Thank you!!! that was really helpful I'll +rep as soon as I'm able to
17. Re: C3 Trig Help
(Original post by 0range)
Thank you!!! that was really helpful I'll +rep as soon as I'm able to
No problem, you are welcome.
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