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Algebra Level 3

Calculate the value of max [ p 2 ] \max\left[p^2\right] if it is known

ln ( 1 ) = p π . \ln(-1)=p\pi.


The answer is -1.

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4 solutions

Tunk-Fey Ariawan
Mar 29, 2014

Note that: i 2 = 1 i^2=-1 and e 2 n 1 2 i π = cos ( 2 n 1 2 i π ) + i sin ( 2 n 1 2 i π ) = i for n is odd . e^{\frac{2n-1}{2}i\pi}=\cos\left(\frac{2n-1}{2}i\pi\right)+i\sin\left(\frac{2n-1}{2}i\pi\right)=i\quad\text{for }n\text{ is odd}. Therefore ln ( 1 ) = ln i 2 = ln ( e 2 n 1 2 i π ) 2 = ln ( e ( 2 n 1 ) i π ) = ( 2 n 1 ) i π . \begin{aligned} \ln(-1)&=\ln i^2\\ &=\ln\left(e^{\frac{2n-1}{2}i\pi}\right)^2\\ &=\ln\left(e^{(2n-1)i\pi}\right)\\ &=(2n-1)i\pi. \end{aligned} Hence, p 2 = ( ( 2 n 1 ) i ) 2 = ( 2 n 1 ) 2 p^2=((2n-1)i)^2=-(2n-1)^2 and max [ p 2 ] \max\left[p^2\right] is obtained when n = 1 n=1 . Thus, max [ p 2 ] = ( 2 ( 1 ) 1 ) 2 = 1 \max\left[p^2\right]=-(2(1)-1)^2=\boxed{\color{#3D99F6}{-1}} .


# Q . E . D . # \text{\# }\mathbb{Q.E.D.}\text{ \#}

Well, you should have asked the maximum value of p 2 p^2 , as p p can also be 3 i , 5 i , 7 i , 9 i , 3 i , 5i , 7i , 9i ,\dots as 1 -1 can be written as e 3 i π , e 5 i π e^{3i \pi} , e^{5i \pi} \dots

jatin yadav - 7 years, 2 months ago

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Thanks for the correction Jatin. Did you mean 1 -1 can be written as e 2 n 1 2 i π e^{\frac{2n-1}{2}i\pi} for n n is odd?

Tunk-Fey Ariawan - 7 years, 2 months ago

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Well, 1 = e i ( 2 n 1 ) π -1 = e^{i(2n-1)\pi} could have been directly.

jatin yadav - 7 years, 2 months ago

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@Jatin Yadav I might be sleepy when I posted this solution and replied your comment. :D

Tunk-Fey Ariawan - 7 years, 2 months ago

That's how I solved it.

Sharky Kesa - 7 years, 2 months ago

but if p2= x, then can be writtten as -1

Jasveen Sandral - 7 years, 2 months ago

nearly same way QED...

Vikash Bhandari - 7 years, 2 months ago

i am trying to nderstand

Henry Owei - 7 years, 1 month ago
Aravind M
Oct 15, 2014

We can solve it in an easier way... Note that taking e^ taking from both sides' we get e^pπ=-1.... Noiw famous Euler's eq, e^ix= coax + isinx,, and comparing, we get p=I= sqrt(-1)....$==> p^2= -1...

Given that ln ( 1 ) = p π e p π = 1 \ln{(-1)} = p \pi\quad \Rightarrow e^{p\pi} = -1

This implies that p = i ( 2 n + 1 ) p = i(2n+1) , where n = 0 , 1 , 2 , . . . n = 0, 1, 2, ... and i = 1 i = \sqrt{-1} . This is because e i ( 2 n + 1 ) π = cos ( 2 n + 1 ) π + i sin ( 2 n + 1 ) π = 1 e^{i(2n+1)\pi} = \cos {(2n+1)\pi} + i \sin {(2n+1)\pi} = -1

Since p 2 = i 2 ( 2 n + 1 ) = ( 2 n + 1 ) p^2 = i^2(2n+1) = -(2n+1) , this means that m i n [ p 2 ] = 1 min[p^2] = \boxed {-1} , when n = 0 n=0 .

Saurav Sharma
Mar 29, 2014

as ln(-1)= 3.14i so just sustitute this value in the given equation we will get the answer

e ( i × π ) = 1 e^(i\times{\pi})=-1

Ali Fathi - 7 years ago

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