2017 OCR C1 official thread - 17th May

thanks. are u sure this is a c1 paper? the paper number is 4721

I just noticed in my CGP book for OCR it has two more circle theorems - "the perpendicular from the centre to a chord bisects the chord" and " a radius and tangent to the same point meet at right angles".

Seriously, the arithmetic is the one part of C1 that means I only get 75-80 UMS on average when I want to get 90 UMS. It's seriously annoying! Especially in factorisation questions. I can't use complete the square because of too many fractions. For example,

3x^2-20x+28=0

(C1 June 2015 OCR Question 6)

I somehow got 3x^12x-8x+28 before realising 12*8 is 96. The mark scheme said it was supposed to be 14*6=28.

I looked at the examiners' report and they said "those who did found it difficult to factorise. Attempts to use the formula were also hampered by the relatively large number 28 and so many candidates got no further."

How can I fix my arithmetic problem before Wednesday? If I can't fix it, it'll mean I'll be spending 15 minutes on a 5 mark question to ensure my arithmetic is perfect. I can do factorisation easily when the numbers aren't too difficult, I under the process perfectly, it's just my bad mental arithmetic that lets me down.

Let me see if I can help.

So for factorising a quadratic equation where the x^2 coefficient is greater than 1 I always multiply the coefficient by the term without the x and then find two values that multiply to give this value and add or subtract to give the original x value. I then factorise this adn can work out the x values. In your question for example,

3x^2 - 20x + 28 = 0 (I multiply the 3 in front of the x^2 by 28 to get 84)
3x^2 - 6x - 14x +28 = 0 (Found two values which sum together to give -20x and which multiply together to give 84)
3x(x-2) - 14(x-2) = 0 (I have taken out common factors and need to make sure the term inside the bracket is the same for each which it is)
(3x-14)(x-2) = 0 (I've taken the terms that were not previously in a bracket and put them in a bracket along with the other bracket, (x-2))
Therefore x = 14/3 or x = 2 (when brackets each equal zero)
Sub into original equations to find y and you're done! Hope this helps.

Thanks, but I know how to factorise. My problem is mainly:
"Found two values which sum together to give -20x and which multiply together to give 84"

How do I do fast, accurate mental arithmetic to save time in the exam?

Yh sorry, I'm not too sure. I personally just think about what numbers I can clearly see divide into the number and note them down until I see a match. However I do not know of a technique to save time, but I guess you could practice it and do it fairly fast. Sorry that I couldn't be of great assistance.

Have you done question 9ii on the June 2011 paper (http://www.ocr.org.uk/Images/62239-question-paper-unit-4721-core-mathematics-1.pdf)? I have no idea how to do it, even after drawing a diagram.

On question 10iii of this paper http://www.examsolutions.net/papers/solomon/OCR/C1/C1D.pdf?x65970, I did gradient of the perpendicular is 1/2 and so the equation of the bisector is y+2 = 1/2 (x−5). I then rearranged and subbed into (5-x)/(-2-y) = 1/2, but that gave me the wrong answer.

In the mark-scheme (see: http://www.examsolutions.net/papers/solomon/OCR/C1/C1DMS.pdf?x65970) they said y = 0 and I have no idea where that came from. Does anyone know how to solve this?

Your rearrangement should equal 2 not 1/2. The perpendicular bisector goes through the centre at point (x,0). y=0 because we know the centre lies on the x-axis, all you gotta do is solve for x.

How do you know that the centre is on the x-axis?

Read the question again

do yous think this core1 paper will be harder or easier than last year, 2016 paper was canny solid not gonna lie boys

Harder as teachers said this year is the last cohort to sit maths AS level exams

do yous think this core1 paper will be harder or easier than last year, 2016 paper was canny solid not gonna lie boys