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Mathematicians who do OCR MEI help

How many significant figures should you use? I've seen people say always use 3 s.f. unless the question says otherwise but in this paper

http://mei.org.uk/files/papers/c409ju_as36.pdf

the very last question on paper A 8 iii) says you have to give your answer to 4 s.f. so does anyone know

And if someone could explain this: Q5 ii) http://mei.org.uk/files/papers/c410ju_weio.pdf

How does the mark scheme go from Integral 3dy/(y-2)(y+1) =....

To Integral (1/(y-2) - 1/y+1)dy=.....

How did they get rid of the 3 and turn the product into a subtraction I dont understand?

(edited 8 years ago)

Reply 1

Kvothe the Arcane

Original post by statskid77

Their marking guidance says

Wrong or missing units in an answer should not lead to the loss of a mark unless the scheme specifically indicates otherwise. Candidatesare expected to give numerical answers to an appropriate degree of accuracy, with 3 significant figures often being the norm. Smallvariations in the degree of accuracy to which an answer is given (e.g. 2 or 4 significant figures where 3 is expected) should not normally bepenalised, while answers which are grossly over- or under-specified should normally result in the loss of a mark. The situation regardingany particular cases where the accuracy of the answer may be a marking issue should be detailed in the mark scheme rationale. If indoubt, contact your Team Leader.

bump

Original post by statskid77

Generally when they state 'giving your answers in decimal' or 'approximate' i always use 4.

To address the second question, they have used partial fractions from the first section, and then used that result to integrate the previous statement. Check Q5 i).

Reply 4

Zacken

Original post by statskid77

And if someone could explain this: Q5 ii) http://mei.org.uk/files/papers/c410ju_weio.pdf

How does the mark scheme go from Integral 3dy/(y-2)(y+1) =....

To Integral (1/(y-2) - 1/y+1)dy=.....

How did they get rid of the 3 and turn the product into a subtraction I dont understand?

I'm not an OCR MEI mathematician, but, as above, in part 5(i), you proved that

\displaystyle \frac{3}{(y-2)(y+1)} \equiv \frac{1}{y-2} - \frac{1}{y+1}

so in your DE:

\displaystyle \int \frac{\mathrm{d}y}{(y-2)(y+1)} = \int x^2 \, \mathrm{d}x \iff \int \frac{3 \, \mathrm{d}y}{(y-2)(y+1)} = \int 3x^2 \, \mathrm{d}x

Then apply the partial factorisations you found in part 5(i).

Reply 5

blipson

Does anyone know on this C3 paper why on question 8iii (last question) the mark scheme says sin(x+pi) =-sinx ?

I was thinking of using a C4 method to expand the brackets but surely that wouldn't be right as this is a C3 exam? help much appreciated btw

http://www.mei.org.uk/files/papers/c306ju_kw7e.pdf

Reply 6

HapaxOromenon3

Original post by blipson

You could expand it out using the addition formula, but for a C3 method, consider the graph of y = sin x. Notice that it has rotational symmetry so e.g. the values of y from x=pi to x=2pi are the same as the values from x=0 to x=pi, just made negative and in the reverse order.