Is this allowed in HL Maths LC?

jos360

Ok so I was just doing this question (see attached image) (its off the 1999 paper)

Its basically (n+4)C(2)=91 If my notations ok (hopefully boards will be more forgiving of bad notation than http://www.physicsforums.com/ is )

So I used the 'binomial coefficients' rule on page 20 of the log tables to get:
(n+4)! / 2!(n+4-2)! =91 which I worked to (n+4)! / (n+2) = 182.

Im not sure if thats correct or not because Im learning these chapters myself. But what I'm wondering is, is what I did next ok because I didn't have any idea how to work with unknowns in factorials.

On my calculator (Casio fx83GT Plus) I pressed:
AC, MODE, 3 (TABLE), and typed in f(x)= (x+4)!/(x+2)!. The I pressed =, for START? I used 5 and for END? I used 15 and set STEP? to 1{ I knew the answer would be roughly 7-12 }. The I scrolled down the table to where f(x)=182 and the x value was 10. And 10 is the correct answer

I used this in my mocks for one question and it's really handy if you know the approximate answer which I do a lot because I really like theoretical maths but not applied maths My teacher has always given me full marks for this even when I admitted how I did it.

So am I ok to do this if im stuck in the LC or will the corrector penalize me for going from (n+4)! / (n+2)! = 182 => n= 10 ?

My teacher said if you get the right answer its full marks and the working outs are only important if you slip/blunder to get the 7/10 marks.


Anyone know?

Canard

I'm not 100% sure because I havent done that, but in my mock we hadnt done sequences & series but the (a) part was something like 10 C N = 45, find N, and I got it by trial and error and got the full 10 marks - so it seems theres no problem.

jos360

Thanks bro,

Yea it seems far to easy because you can solve half the paper using the TABLE function on the calculator

But the department know the calculators capabilities when they put it on the list of allowed calculators for the exams so I guess it could be allowed.

I just wanna be really sure before I risk something like it in my LC

Reading the manual that came with my calculator; best investment on 15 min study ever!

deconduo

I'm pretty sure when I did the LC we were only allowed used standard scientific calculators. No graphing ones were allowed. You also left out a pretty big step when solving it:



Solving that gives x=10, x=-17.

If I was marking it I would definitely consider that as a large enough omission to count as a blunder (-3 marks).

jos360

deconduo wrote: »

I'm pretty sure when I did the LC we were only allowed used standard scientific calculators. No graphing ones were allowed. You also left out a pretty big step when solving it:



Solving that gives x=10, x=-17.

Oh that would be indeed awkward

Ok so in that case it obviously dosn't get the correct answer
We still cant use graphing calculators for exams, but our 'scientific' calculators can render a table of points of a curve which is basically the same thing and there are other parts of the syllabus where we a supposed to do that.

Guess the best thing to do is brush up on my factorials so

If I was marking it I would definitely count that as a large enough omission to count as a blunder (-3 marks).

Oh I would be very happy to take a blunder there, usually for only finding one value when theres two you only get an attempt mark

Thanks:)

deconduo

jos360 wrote: »

Oh that would be indeed awkward

Ok so in that case it obviously dosn't get the correct answer
We still cant use graphing calculators for exams, but our 'scientific' calculators can render a table of points of a curve which is basically the same thing and there are other parts of the syllabus where we a supposed to do that.

Guess the best thing to do is brush up on my factorials so



Oh I would be very happy to take a blunder there, usually for only finding one value when theres two you only get an attempt mark

Thanks:)

Well -17 isn't an answer, as n has to be positive. Final line you would write would be:



You did get the right answer, but trial and error isn't a valid way to get there.