Question

520 logs are stacked in the following manner : 20 logs in the bottom row. 2 logs less in the next row, 2 logs less in the row next to it and so on. In how many rows are the 520 logs placed? How many logs are in the top row?

Answer 1

It is an AP such that a = 20 & d= -1

Sⁿ= 200

= 200 =n/2 [2(20)+(n-1)(-1)]

=400= n[40+1-n]

=n²-41n+400 = 0

(n-25) (n-16) = 0

So n= 16 or n = 25

T₁₆=a+15d = 20 - 15 = 5

T₂₅= a+24d = 20 - 24 = -4

Since T₂₅ is not possible.

∴ n= 16 & 5 logs are placed in the top row

Answer 2

Answer:

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