# C3: OCR Mei ProofsWatch

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#1
Could anyone help with proof questions in C3. I was just wondering how people would go about doing them? Also if anyone could just list me some facts about numbers that would help me solve proofs that would great! For instance the idea that multiplying two odd numbers always gives an odd number etc.

Thanks.
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5 years ago
#2
(Original post by Mrbrian)
Could anyone help with proof questions in C3. I was just wondering how people would go about doing them? Also if anyone could just list me some facts about numbers that would help me solve proofs that would great! For instance the idea that multiplying two odd numbers always gives an odd number etc.

Thanks.
I'm afraid your request is so impossibly vague that it's going to be very difficult for anyone to give you any real help.

It would be better if you gave us an example of a specific question that you are finding difficult so we can focus on that.
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#3
(Original post by davros)
I'm afraid your request is so impossibly vague that it's going to be very difficult for anyone to give you any real help.

It would be better if you gave us an example of a specific question that you are finding difficult so we can focus on that.
My apologies, but ok for example Q7 part 2

http://www.mei.org.uk/files/papers/2011_June_c3.pdf

Making reference to the mark scheme how do you know the product is divisible by 8??
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5 years ago
#4
(Original post by Mrbrian)
My apologies, but ok for example Q7 part 2

http://www.mei.org.uk/files/papers/2011_June_c3.pdf

Making reference to the mark scheme how do you know the product is divisible by 8??
Ok so 3^2n-1 = (3^n+1)(3^n-1)

We know 3^n is always odd (odd*odd=odd) therefore 3^n-1 and 3^n+1 are consecutive even numbers (think about it, 3-1=2 3+1=4, 2 and 4 are consecutive even numbers). With consecutive even numbers one of them always has a factor of 4 (again think about this) and if we multiply a number with a factor of 4 and a factor of 2 (the other even number) the product will always be divisible by 8.

This was a fairly tough proof question, considering they hardly have anything on the website to teach proof and it's definitely the hardest topic in C3 I'm not surprised people struggle!
1
#5
(Original post by Davelittle)
Ok so 3^2n-1 = (3^n+1)(3^n-1)

We know 3^n is always odd (odd*odd=odd) therefore 3^n-1 and 3^n+1 are consecutive even numbers (think about it, 3-1=2 3+1=4, 2 and 4 are consecutive even numbers). With consecutive even numbers one of them always has a factor of 4 (again think about this) and if we multiply a number with a factor of 4 and a factor of 2 (the other even number) the product will always be divisible by 8.

This was a fairly tough proof question, considering they hardly have anything on the website to teach proof and it's definitely the hardest topic in C3 I'm not surprised people struggle!
This exact question came up pretty much haha, "prove that two consecutive even numbers are divisable by 8"

Thank you.
0
5 years ago
#6
(Original post by Mrbrian)
This exact question came up pretty much haha, "prove that two consecutive even numbers are divisable by 8"

Thank you.
I know I did it! Woo!
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#7
(Original post by Davelittle)
I know I did it! Woo!
i was convinced i did everything right when i came out, and turns out i stupidly used a wrong value in my integration by parts at the end. Other than that i think my method was correct how many marks do you think I'd loose??? And how many marks do you recon is 90 ums?
0
5 years ago
#8
(Original post by Mrbrian)
i was convinced i did everything right when i came out, and turns out i stupidly used a wrong value in my integration by parts at the end. Other than that i think my method was correct how many marks do you think I'd loose??? And how many marks do you recon is 90 ums?
maybe 1-3 idk mate sorry haha!

Hmm hopefully 64 but could be up to 66 I guess, again I'm not sure how they scale the UMS at higher scores
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