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diff/ eq of curve / tangent etc My workings not the same as textbook, please help

I must be doing the mistake over and over, I need a fresh pair of eyes to catch my blunder and a bigger brain....:-)

See the questions and my attempts in amages
Reply 1
anyone please..........
Reply 2
First one you seem to have changed a -2 to a 2 in the exponent.


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Reply 3
I see now I made the same mistake in the 2 questions I multiplied by -4 to clear the denominator in the fraction but then for some reason changed - 4x^-2 to 16x^2 as well These stupid mistakes drive me crazy, but I enjoy the challenge:biggrin:
Reply 4
pheeeeew that was a long mistake but a worthwhile one I went round the world to sort this one in my thick head......but all sorted now.
Reply 5
Original post by silverpuma
pheeeeew that was a long mistake but a worthwhile one I went round the world to sort this one in my thick head......but all sorted now.


But to be on the safe side, I realised after I said I had it sorted I still wasn't right so after a bit of head scratching I think I understand my mistake. And I now can arrive at the right answer as the textbook said.

....But......just in case, I wonder would you have another look at my new working out especially the area inside the black box on top right side of page as this was where my trouble was. I'd be really grateful if you could be critical as I want to kelp doing this the "best" or most efficient way I can and not be sloppy in my working out method, so be ruthless;-) It still seems to be that there is probably a more elegant way.
(edited 8 years ago)
Reply 6
Looks good to me.

Practice it more and the multiplying up stage will be a single line. You can just "see" that you get 16 on one side and 9x^2 on the other.

You could also go the crazy route and square-root the entire equation from step 1 to find that: 2/x = 3/2, so x = 4/3. Of course you lost the negative answer when taking the positive square root, so you know x = -4/3 is also a solution. But that's a bit mental and not advisable in an exam with ECFs.
Reply 7
Original post by mik1a
Looks good to me.

Practice it more and the multiplying up stage will be a single line. You can just "see" that you get 16 on one side and 9x^2 on the other.

You could also go the crazy route and square-root the entire equation from step 1 to find that: 2/x = 3/2, so x = 4/3. Of course you lost the negative answer when taking the positive square root, so you know x = -4/3 is also a solution. But that's a bit mental and not advisable in an exam with ECFs.


Thanks Mik1a for taking the time to reply and making the suggestions

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