1. ## limit n-->infinty

Hi my friends
can you help to find the limit of the following equation

Area = $\displaystyle \lim_{n \rightarrow \infty}$ $\displaystyle \frac{\pi}{n} *sin(\frac{\pi*n+\pi}{2})$

2. Well think about it. $\displaystyle \left|\sin\left(\frac{\pi(n+1)}{2}\right)\right|$ is always less than or equal to $\displaystyle 1$, and $\displaystyle \frac{\pi}{n}$ gets as small as you wish...

3. Originally Posted by metallica007
Hi my friends
can you help to find the limit of the following equation

Area = $\displaystyle \lim$ n--> $\displaystyle \infty$ $\displaystyle \frac{\pi}{n} *sin(\frac{\pi*n+\pi}{2})$
Well u can use the squeeze theorem.

-1<= sinx<=1
so
lim n => inf -pi/n<= pi/n *sinx<=pi/n
From this, is pretty obvious what the answer is...

I did not get it
i know this :
lim n--> infinity pi/n =0
lim n--> infinity $\displaystyle sin(\frac{\pi*n+\pi}{2})$ does not exist
so the answer is ... 0 !!!

please correct me if i am wrong

5. Originally Posted by metallica007
I did not get it
i know this :
lim n--> infinity pi/n =0
lim n--> infinity $\displaystyle sin(\frac{\pi*n+\pi}{2})$ does not exist
so the answer is ... 0 !!!

please correct me if i am wrong
Sin always exist between -1 and 1...

6. but the answer doesn't seems right to me
because i tried to find the area of sin(x) from 0 to pi using limit --> infinity and Riemann sums and i got 0
but when i tried to integrate it using integration rules and i get 2 !!

7. i am , doing it like this
$\displaystyle \Delta x =$$\displaystyle \frac{\pi}{n}$

$\displaystyle \lim n-->\infty \sum Sin(xi) \Delta x$

$\displaystyle \lim n-->\infty\\\\ Sin(\frac{\pi}{n} \frac{n(n+1)}{2}) \frac{\pi}{n}$

$\displaystyle \lim$ n--> $\displaystyle \infty$ $\displaystyle \frac{\pi}{n} *sin(\frac{\pi n+\pi}{2})$

I dont know if that was the right steps to find area using Riemann sum and limit

please correct me if i am wrong

8. 0... The answer is 0...

9. Originally Posted by VonNemo19
when a =0 , b = pi

does $\displaystyle \int\sin(x)= 0$ !!!!

10. Originally Posted by metallica007
when a =0 , b = pi

does $\displaystyle \int\sin(x)= 0$ !!!!
No, actually its 1

11. Originally Posted by VonNemo19
No, actually its 1
2

12. Guys , what about my question
why i get 0 instead of 2 ( or 1 ) !!!!

13. Originally Posted by metallica007
Guys , what about my question
why i get 0 instead of 2 ( or 1 ) !!!!
as stated before ...

$\displaystyle -1 \le \sin(anything) \le 1$

so ...

$\displaystyle \lim_{n \to \infty} \, \frac{\pi}{n}(a \, value \, stuck \, between \, -1 \, and \, 1) = 0$

now look at the graph

14. Originally Posted by metallica007
can you help to find the limit of the following equation
Area = $\displaystyle \lim _{n \to \infty }\frac{\pi}{n} *sin(\frac{\pi*n+\pi}{2})$
It seems to me that this thread is very confused.
It seem that the OP assumes that the above limit is an approximating sum for $\displaystyle \int\limits_0^\pi {\sin (x)dx}$.
But that is totally wrong. A correct approximating sum (righthand sum) is $\displaystyle \frac{\pi }{n}\sum\limits_{k = 1}^n {\sin \left( {\frac{\pi }{n}k} \right)}$.
Thus we want this limit: $\displaystyle \lim _{n \to \infty } \frac{\pi }{n}\sum\limits_{k =1 }^n {\sin \left( {\frac{\pi }{n}k} \right)}$.

Page 1 of 2 12 Last