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

  • Thread starter Daaniyaal
  • Start date
  • #1
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Homework Statement


How many terms are needed in the series 11+16+21+26...+ to exceed 450
d=5


Homework Equations


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


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 :(
 

Answers and Replies

  • #2
SammyS
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Homework Statement


How many terms are needed in the series 11+16+21+26...+ to exceed 450
d=5


Homework Equations


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


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 :(
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
64
0
I solved the quadratic and got the right answer, thanks!
 

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