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C4 Maths - surely one of the hardest questions....!! Genius req'd please!

Here is the paper:

http://www.madasmaths.com/archive/iygb_practice_papers/c4_practice_papers/c4_t.pdf

The part I can't do is Question 1, part b) ii)

Here are the solutions:

http://www.madasmaths.com/archive/iygb_practice_papers/c4_practice_papers/c4_t_solutions.pdf

My question is: In line 7-8 of the working for part b) ii) .... why is it necessary to cube every term; how can doing so possibly give a different result than would simply proceeding from line 7 onwards using the values of y and dy/dt given on the page ? Baffled.

Thanks!!!
Original post by Jameshyland
Here is the paper:

http://www.madasmaths.com/archive/iygb_practice_papers/c4_practice_papers/c4_t.pdf

The part I can't do is Question 1, part b) ii)

Here are the solutions:

http://www.madasmaths.com/archive/iygb_practice_papers/c4_practice_papers/c4_t_solutions.pdf

My question is: In line 7-8 of the working for part b) ii) .... why is it necessary to cube every term; how can doing so possibly give a different result than would simply proceeding from line 7 onwards using the values of y and dy/dt given on the page ? Baffled.

Thanks!!!


I imagine cubing was to simplify the workings so that the cube root could be taken at the end and it is easier to work without using cube roots all the way through.

I did it without cubing though and got the same result :smile:

20170605_203746.jpg

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