Don't guess

Algebra Level 3

89 , 81 , 73 , … \large 89 , 81 , 73 , \dots

Find the least number of terms that can be taken from the sequence starting from the 1st term to make the sum negative


The answer is 24.

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Rawan Medhat by Rawan Medhat , 22, Egypt

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1 solution

Annanya Gupta
Mar 1, 2016

Using the formula for a sum of an A.P.,

n(178 + (n -1)(-8))/2 <0

186 - 8n <0

23.25<n

Hence, the least value of n = 24

1 Helpful 0 Interesting 0 Brilliant 0 Confused

186/8=23.25, so answer is 24.

Kushagra Sahni - 5 years, 3 months ago

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