calculus I question?

2020-08-11 6:51 pm
by induction we can prove , (x+x....+x), x times= x^2....eg. 4+4+4+4 =16 and 5+5+5+5+5=25...so if we take derivative of both sides we get (1+1+1..+1) x times = 2x..but (1+1+1..).x times =x,   thus 2x=x for all x.......what is wrong ?

回答 (4)

2020-08-11 7:07 pm
The stepsin your calculation:
Step [1]: (1 + 1 + 1 + …… + 1) x = x²
Step [2]: Take derivative on both sides:  1 + 1 + 1 + …… + 1 = 2x
Step [3]: x = 2x ? …… [3]

The reason of wrong calculation:
From step [1] to step [2]: (1 + 1 + 1 + …… + 1) was treated as a constant.
From step [2] to step [3]: (1 + 1 + 1 + …… + 1) was treated as variable x.

Actually, (1 + 1 + 1 + …… + 1) = x and thus it a variable.
The derivative on the left side is wrong.
2020-08-12 1:15 am
The only thing wrong is that you do not even know the basic axioms of arithmetic, so how you reckon to do calculus is a mystery 

(x+x....+x), x times= x^2 ?  NO IT DOESN'T.

Learn the basics and start over.
2020-08-11 7:17 pm
deriv. of x^3  =  3x^2
derv. of x^5  =  5x^4

if we take derivative of both sides we get (1+1+1..+1) x times = 2x
 ...  as you said ...  this is a derivative, so what is your math. reasoning for setting this equal to the original equation?  ...  there is NONE.
  .but (1+1+1..).x times =x   <<<  this is the orig. 'equation' 
 ....  why are you setting the original equal to a derivative???  ==  makes no math sense.

  ALSO
   (2 + 2)  =  2^2  ....  and  (3 + 3 + 3)  =  3^2
  .but (1+1+1..).x times =x,  in this case x times means to sum 1 exactly 1 time so it is really (1 + nothing)  = 1^2 = 1
    so     1  =  1^2   ....  correct
     deriv.  ???  left .. deriv of  1  =  0  <<<  deriv of a constant is 0
     deiv. of right  =  ... deriv of 1^2??  still a constant .. =  0  not 2x .. this equation has no x variable
2020-08-11 7:10 pm
what is wrong ?
Is that you really don't have two equation which are 
equal? 
You have one equation which is equal to another 
only when x has a specific value.  

you are  
if 
f(x) =ax     
g(x) = x^2      

f(a) = a^2 
g(a) = a^2   
they are equal  only when   the x value  = a  
That is only that have shown

ax=x^2  
x^2 -ax  = 0 
x(x-a) = 0 
so this is only true when x =0 or x = a  

they are not equal all the time.     

since they are only equal when x =a  or x = 0 
then why should their derivatives be equal. 


收錄日期: 2021-04-24 07:58:04
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20200811105158AAhKGe9

檢視 Wayback Machine 備份