# d(x^2)/dx = x

THUDandBLUNDER
Posers and Puzzles 06 Jun '04 09:40
1. 06 Jun '04 09:40
d(x^2)/dx = x

PROOF:

d(x^2)/dx = d(x + x + x + ....... (x times))/dx

= d[x]/dx + d[x]/dx + d[x]/dx....... (x times)

= 1 + 1 + 1 + ....... (x times)

= x

QED

😉
2. TheMaster37
Kupikupopo!
06 Jun '04 16:511 edit
Now for integer (non-positive), rational (non-integer) or real (non-rational) x
3. 07 Jun '04 06:111 edit
Originally posted by TheMaster37
Now for integer (non-positive), rational (non-integer) or real (non-rational) x
I guess 'Now...' should be &quot;Not...'

But hey, I'm not greedy - if it holds for the positive integers, that's OK by me. 😏

.
4. TheMaster37
Kupikupopo!
07 Jun '04 17:58
😉

I could point out that d/dx is defined with limits, the quotient of the horizontal and vertical displaccement in a graph, with the horizontal displacement going to 0. Since that limit process is kinda abrubt on the integers; 3,2,1,0,0,0,0... you divide by 0 and can get any answer you like.

Assuming the limit process is correctly defined for natural numbers...
5. Acolyte
07 Jun '04 19:07
How about this: let f(x) = x*[x], where [x] is the integer of smallest modulus which has greater modulus than x.

If x isn't an integer, f'(x) = [x]
OTOH if g(x) = x^2, g'(x) = 2x

So on the set of positive reals which aren't integers, f and g are differentiable and g' - f' tends to infinity as x tends to infinity, but f &gt; g for all x. (*)

This is a demonstration of why your domain must be connected if you want differentiable (and indeed continuous) functions to behave themselves. For example, it's not possible to find two functions which satisfy (*) on all the positive reals.
6. TheMaster37
Kupikupopo!
07 Jun '04 20:22
I'm a litle bit in the dark on what you mean by modulus...[x] is usually the entier-function; [x] is the greatest integer no exceeding x (a sort of rounding down).
7. 09 Jun '04 13:27
Originally posted by THUDandBLUNDER
d(x^2)/dx = x

PROOF:

d(x^2)/dx = d(x + x + x + ....... (x times))/dx

= d[x]/dx + d[x]/dx + d[x]/dx....... (x times)

= 1 + 1 + 1 + ....... (x times)

= x

QED

😉
Brillient
8. 09 Jun '04 13:28
Originally posted by crec2k
Brillient
That was a purposeful mistake.
FYI, myles is fat.
9. 09 Jun '04 13:29
Originally posted by crec2k
That was a purposeful mistake.
FYI, myles is fat.
It is true, i am very fat.😛