Results 1 to 13 of 13

Math Help - infinite series problem

  1. #1
    Newbie
    Joined
    Mar 2010
    Posts
    14

    infinite series problem

    Find the sum of each of the geometric series given below. For the value of the sum, enter an expression that gives the exact value, rather than entering an approximation. A.

    I got -4 which is wrong.
    Any thought on that one ?
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor
    skeeter's Avatar
    Joined
    Jun 2008
    From
    North Texas
    Posts
    11,623
    Thanks
    428
    Quote Originally Posted by dylan5188 View Post
    Find the sum of each of the geometric series given below. For the value of the sum, enter an expression that gives the exact value, rather than entering an approximation. A.

    I got -4 which is wrong.
    Any thought on that one ?
    you found the sum of an infinite geometric series ... this one is finite.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Newbie
    Joined
    Mar 2010
    Posts
    14
    Quote Originally Posted by skeeter View Post
    you found the sum of an infinite geometric series ... this one is finite.
    yea I used way from textbook. In this problem, r is -1/2. which has aabs value<1.
    and put in to a/(1-r) then I got -4.
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Super Member Anonymous1's Avatar
    Joined
    Nov 2009
    From
    Big Red, NY
    Posts
    517
    Thanks
    1
    -6+3-3(\frac{1}{2}+...+\frac{1}{2^{11}} ) = -3(1+(\frac{1}{2} +...+\frac{1}{2^{11}} ))= -3(1+(2^{10} +...+2+1)/2^{11})...

    Figure out what 2^{10} + 2^9+...+2 + 1 is and your pretty much done.
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Newbie
    Joined
    Mar 2010
    Posts
    14
    Quote Originally Posted by Anonymous1 View Post
    -6+3-3(\frac{1}{2}+...+\frac{1}{2^{11}} ) = -3(1+(\frac{1}{2} +...+\frac{1}{2^{11}} ))= -3(1+(2^{10} +...+2+1)/2^{11})...

    Figure out what 2^{10} + 2^9+...+2 + 1 is and your pretty much done.
    I just tried it, still couldn't get it right.
    Follow Math Help Forum on Facebook and Google+

  6. #6
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by dylan5188 View Post
    Find the sum of each of the geometric series given below. For the value of the sum, enter an expression that gives the exact value, rather than entering an approximation. A.

    I got -4 which is wrong.
    Any thought on that one ?
    a = -6, r = -1/2 and n = 13. Substitute into the usual formula. Please show your work and say where you're stuck if you need mor help.
    Follow Math Help Forum on Facebook and Google+

  7. #7
    Super Member Anonymous1's Avatar
    Joined
    Nov 2009
    From
    Big Red, NY
    Posts
    517
    Thanks
    1
    Quote Originally Posted by dylan5188 View Post
    I just tried it, still couldn't get it right.
    What do you mean? You couldn't figure out 2^10+... thing?

    While I have MATLAB open

    EDU>> 2^10+2^9+2^8+2^7+2^6+2^5+2^4+2^3+2^2+2+1

    ans =

    2047

    So your answer is -5.9985...

    Unless I made an algebra mistake somewhere, but I can't see one.
    Follow Math Help Forum on Facebook and Google+

  8. #8
    Newbie
    Joined
    Mar 2010
    Posts
    14
    Quote Originally Posted by mr fantastic View Post
    a = -6, r = -1/2 and n = 13. Substitute into the usual formula. Please show your work and say where you're stuck if you need mor help.
    yea I got r is -1/2. which has a abs value<1.
    and put in to a/(1-r) then I got -4. I didnt use n though
    Follow Math Help Forum on Facebook and Google+

  9. #9
    Newbie
    Joined
    Mar 2010
    Posts
    14
    Quote Originally Posted by Anonymous1 View Post
    What do you mean? You couldn't figure out 2^10+... thing?
    I meant I tried your way and got -5.99.... from calculator, but when I submit this answer, is shows answer incorrect.
    Follow Math Help Forum on Facebook and Google+

  10. #10
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by dylan5188 View Post
    yea I got r is -1/2. which has a abs value<1.
    and put in to a/(1-r) then I got -4. I didnt use n though
    You were told all the way back in post #2 that you had the wrong formula!

    Your series is NOT infinite, it is FINITE. Go to your class notes or textbook, find the formula for a finite geometric series and substitute the appropriate values into it.
    Follow Math Help Forum on Facebook and Google+

  11. #11
    Super Member Anonymous1's Avatar
    Joined
    Nov 2009
    From
    Big Red, NY
    Posts
    517
    Thanks
    1
    OOO its alternating oops.

    Try -4.0005
    Follow Math Help Forum on Facebook and Google+

  12. #12
    Newbie
    Joined
    Mar 2010
    Posts
    14
    Quote Originally Posted by mr fantastic View Post
    You were told all the way back in post #2 that you had the wrong formula!

    Your series is NOT infinite, it is FINITE. Go to your class notes or textbook, find the formula for a finite geometric series and substitute the appropriate values into it.

    Finally got it. I didnt notice is finite, the whole chapter is about infinite.
    Thanks
    Follow Math Help Forum on Facebook and Google+

  13. #13
    Newbie
    Joined
    Mar 2010
    Posts
    14
    Quote Originally Posted by Anonymous1 View Post
    OOO its alternating oops.

    Try -4.0005
    Yea that's the right answer.
    Thanks
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Infinite series problem?
    Posted in the Calculus Forum
    Replies: 1
    Last Post: April 18th 2011, 03:33 AM
  2. Infinite Series Problem
    Posted in the Calculus Forum
    Replies: 4
    Last Post: March 16th 2010, 06:00 PM
  3. Infinite series problem
    Posted in the Calculus Forum
    Replies: 3
    Last Post: March 2nd 2010, 08:02 AM
  4. infinite series problem with log
    Posted in the Calculus Forum
    Replies: 1
    Last Post: April 25th 2009, 01:15 PM
  5. Infinite Series Problem
    Posted in the Calculus Forum
    Replies: 6
    Last Post: April 5th 2008, 11:39 AM

Search Tags


/mathhelpforum @mathhelpforum