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How many terms are needed in the series 11+16+21+26 + to exceed 450

  1. Dec 24, 2012 #1
    1. The problem statement, all variables and given/known data
    How many terms are needed in the series 11+16+21+26...+ to exceed 450
    d=5


    2. Relevant equations
    Sn=n/2(2u1+(n-1)d)


    3. The attempt at a solution
    (455 because is term exceeding 450 by the d of 5)
    455=n/2(22+5n-5)
    455=17n+5n2
    455/17=5n2

    Calculator work,
    but the answer I get is not 12. The textbook says the answer is 12 :(
     
  2. jcsd
  3. Dec 24, 2012 #2

    SammyS

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    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    How do you go from 455=17n+5n2, which is correct,

    to 455/17=5n2, which is incorrect?

    You need to solve the quadratic equation, 5n2 + 17n -455 = 0 .
     
  4. Dec 24, 2012 #3

    jedishrfu

    Staff: Mentor

    Yeah, I get 910= n (17 +5n) which 5n^2 +17n - 910 = 0
     
  5. Dec 24, 2012 #4
    I solved the quadratic and got the right answer, thanks!
     
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