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    Hi

    I was going through a C3 paper (OCR MEI, June 2009) and came across this question attached below.

    Please could somebody explain part (ii)? I proved the first part correctly, but I have no idea how to then go on to prove the second part… the mark scheme says that I have to prove the first parts to be positive? I still don't get it ahhh

    Would really appreciate if anybody could help! Thanks
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    (Original post by Babs Posh)
    Hi

    I was going through a C3 paper (OCR MEI, June 2009) and came across this question attached below.

    Please could somebody explain part (ii)? I proved the first part correctly, but I have no idea how to then go on to prove the second part… the mark scheme says that I have to prove the first parts to be positive? I still don't get it ahhh

    Would really appreciate if anybody could help! Thanks
    x^3>y^3\iff x^3-y^3>0

    So you need to prove that

    x^3-y^3=(x-y)(x^2+xy+y^2)>0

    To do this you'll need to prove that both brackets are >0. Think about how you can use (B) for this. Please post your thoughts/working if you get stuck.
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    (Original post by Babs Posh)
    Hi

    I was going through a C3 paper (OCR MEI, June 2009) and came across this question attached below.

    Please could somebody explain part (ii)? I proved the first part correctly, but I have no idea how to then go on to prove the second part… the mark scheme says that I have to prove the first parts to be positive? I still don't get it ahhh

    Would really appreciate if anybody could help! Thanks
    look at the second bracket in (A),  \displaystyle x^2 +xy + y^2 , can you think of any way to show this is always positive, possibly with a substitution?
 
 
 
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