Is this allowed in exams for maths?

MichailZ

Hi,

If I was to answer a C2 Trigonometry question, using a rule from C3, would I be given full marks, or would they want me to answer it using the material learned in C2 only?

Thanks,

Michail

You can if you want, unless they state a specific method for you to use. But like person above said you really shouldnt need to use any c3 knowledge, you may overcomplicate it.
Give me an example of such a question where you think c3 knowledge would help.

Reply 3

-G-a-v-

16

If you used the rule from C3 correctly, and came to the correct answer using that method, I can't see that you'd lose many, if any, marks. I can't imagine them setting a question on a C2 paper though that would require using C3 methods. Or even if you could use C3 methods in the C2 question, I can't see how it'd be quicker/neater/simpler, if that's what you were thinking. Also, most ideas from C3 are just extensions of those from C2. Do you have a particular question in mind that you've come across?

Reply 4

MichailZ

OP

The question I had in mind was C2/10E Question 9a and 9b

Express 4(cos3x) - sin(90-3x) as a single trigonometric fraction.

Doing it using the C3 formula sin(A-B) = sinAcosB - cosAsinB is easy and you get to the answer "3cos3x" in no time. I am still thinking about how to do this using the C2 formulas... don't laugh :confused:

And the answer comes up right too.

Michail

Reply 5

MichailZ

The question I had in mind was C2/10E Question 9a and 9b

Express 4(cos3x) - sin(90-3x) as a single trigonometric fraction.

Doing it using the C3 formula sin(A-B) = sinAcosB - cosAsinB is easy and you get to the answer "3cos3x" in no time. I am still thinking about how to do this using the C2 formulas... don't laugh :confused:

And the answer comes up right too.

Michail

since sin(90-x) = cos x (a C2 rule or just look at the graphs):

sin(90-3x) = cos3x

thus 4cos3x - sin(90-3x) = 4cos3x - cos3x = 3cos3x

seems easier to use C2 rule tbh

Reply 6

MichailZ

OP

That's very nice... I couldn't spot that, but I am glad to hear that using the other rule is "legal" too.

Thanks for your advice!

Michail

Reply 7

Dekota

MichailZ

Express 4(cos3x) - sin(90-3x) as a single trigonometric fraction.

Doing it using the C3 formula sin(A-B) = sinAcosB - cosAsinB is easy and you get to the answer "3cos3x" in no time. I am still thinking about how to do this using the C2 formulas... don't laugh :confused:

And the answer comes up right too.

Michail

4(cos3x) - sin(90-3x) = 4cos3x - sin(90-3x)
..............................= 4cos3x - cos3x
..............................= 3cos3x

Because, sin(90-a) = cos[a] which is an identity given in C2.

Reply 8

AlphaNumeric

10

There was a question someone asked here about 2 weeks ago involving complex numbers which was FP1 and I gave a method which involved using re^ix and made short work of the question, but I was then told that re^ix is FP2 or FP3, so they had to do it by expanding out (a+ib)⁴ binomially and doing some simultaneous equations. I forget the specifics of it, but it meant about 5 times the amount of work mine did.

Certainly at university level, if you know a theorem from another course that makes short work of a problem you are allowed to apply it (so long as it's appropriate, no stating results when asked for proof etc).

Reply 9

MichailZ

That's very nice... I couldn't spot that, but I am glad to hear that using the other rule is "legal" too.

Thanks for your advice!

Michail

im sure using the rule you used is fine, but be careful when using rules that you're not 'meant' to know, sometimes you may loose method marks for doing this

Reply 10

rpotter

im sure using the rule you used is fine, but be careful when using rules that you're not 'meant' to know, sometimes you may loose method marks for doing this

I have to disagree, unless the question is specific with a method, you wont loose anything (except perhaps time, if its longer method).
"you're not 'meant' to know" - I dont even see how this can be applied to maths, they will not penalise you for knowing a better perhaps more elegant solution.

Reply 11

Malik

I have to disagree, unless the question is specific with a method, you wont loose anything (except perhaps time, if its longer method).
"you're not 'meant' to know" - I dont even see how this can be applied to maths, they will not penalise you for knowing a better perhaps more elegant solution.

well for instance in M2, you could well get penalised for immediately using a formula that gives the range of a projectile by inputting the angle at which it was launced and the speed at which it was launched, rather than working out the tiem of flight and then using this to find the range. Well thats what my teacher said anyway

Reply 12

rpotter

well for instance in M2, you could well get penalised for immediately using a formula that gives the range of a projectile by inputting the angle at which it was launced and the speed at which it was launched, rather than working out the tiem of flight and then using this to find the range. Well thats what my teacher said anyway

True, but m2 questions are written in such a way that it wouldnt be of much use. The questions are always split into parts, where your forced to calculate the other variables. And like any other question you would get full marks if you showed method to how you derived formula, similary to how you need to rearrange suvat equations for method marks.

Reply 13

sea tea

9

rpotter

well for instance in M2, you could well get penalised for immediately using a formula that gives the range of a projectile by inputting the angle at which it was launced and the speed at which it was launched, rather than working out the tiem of flight and then using this to find the range. Well thats what my teacher said anyway

that's because the types of formula you suggest have simply been derived from uvast equations. if you just quote the more concise one, the examiner has no way of knowing that you can actually manipulate the equations yourself, rather than just write down ones given to you by your teacher.

Reply 14

SsEe

13

I used the vector product from P6 in my P3 exam.
I've also seen mark schemes that suggest you don't lose marks. Like in a FP1 exam it may say you get the method marks for using a C4/FP1 technique, a FP3 technique or "any other method" that does the job.

Reply 15

sea tea

that's because the types of formula you suggest have simply been derived from uvast equations. if you just quote the more concise one, the examiner has no way of knowing that you can actually manipulate the equations yourself, rather than just write down ones given to you by your teacher.

indeed, but that's still using a formula that you are not meant to know

Reply 16

rpotter

indeed, but that's still using a formula that you are not meant to know

Stop using the term "not meant to know", there is only "you should know". If you didnt bother diving straight into the calculations and kept your working in algebra form till the end you would end up with projectile formula. Hence you probably do know but never notice.

Reply 17