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Whats Wrong in this? "2 = 1"

 
 
vinsan
 
Reply Wed 7 Sep, 2005 11:27 am
Method 1: Differentiation Method

Let a^b mean a "raised to" b

1^2 = 1
2^2 = 2 + 2
3^2 = 3 + 3 + 3

so for any X > 0

X^2 = X + X + X + ..... (X times)

Differentiate both sides with resepct to X

2X = 1 + 1 + 1 + ... (X times)

2X = X

2 = 1 .... Cancelling X from both sides as X <> 0


Method 2: Logarithm Method

We know that natural log of 2 i.e. Ln(2) = 0.6931471

As per log series transformation ....

Ln(2) = 1 - (1/2) + (1/3) - (1/4) + (1/5) .....

Ln(2) = (1 + 1/3 + 1/5 ....) - (1/2 + 1/4 + 1/6 ....)

Ln(2) = ((1 + 1/3 + 1/5 ....) + (1/2 + 1/4 + 1/6 ....)) - 2(1/2 + 1/4 + 1/6 + 1/8 ....)

Ln(2) = (1 + 1/2 + 1/3 + 1/4 ...) - (1 + 1/2 + 1/3 + 1/4 ....)

Ln(2) = 0


Now we also know Ln(1) = 0 ........ as e^0=1

So Ln(2) = Ln(1) = 0

e^Ln(2) = e^Ln(1) ......... taking the exponent of both sides

2 = 1 .............. as e^Ln(a) = a


I know for sure that the first solution can be criticized on the differentiation but what about the 2nd one.... Confused
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satt fs
 
  1  
Reply Fri 9 Sep, 2005 12:00 am
Method 1:
For differentiation x must take a real number which is not necessarily an integer. And you did not define the "x times" summation in the case of non-integer value of x.

Method 2:
Infinite series should not change the order of terms to have a well defined limit, unless their terms are all positive (non-negative).
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g day
 
  1  
Reply Sat 10 Sep, 2005 05:24 am
Very cute.

Method 1

for x <> 0 say x = 1 + 1 + 1 (x times) (PS try this with a 1/2 or an irrational number and see if it makes any sense to you and hence why your differentiation fails),

diff LHS and RHS wrt X

1 = 0 + 0 + 0 (x times) - beginning to see the error now?

If you wish to differentiate 1 + 1 + 1 (x times) wrt x then you should use lim h->0 of (f(x+h) - f(x))/h to check your results, then you see the RHS does differentiate to 1 (h/h).

Method 2.

Ouch, you've messed with infinite tails of expressions and presumed changing the order and rate of summation doesn't change the result Smile Wrong!

But lovely questions!
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