Quote:
How did you know that the sum of the terms in the denominator was ?
Sum of n terms of an A.P. = n/2 { 2a+(n-1)d}
...
n/2 { 2x1 + (n-1)2}
: n/2 { 2 + 2n - 2}
: n/2 x 2n = n^2
Because the Sum of the first 50 even numbers can be calculated using the same formula to be 2550 , we get the equation:
2550/n^2 = 51/2
and solving for n we get n = 10.
You could also attack this question by plugging in the values in the answer choices :
Given one choice is 10 , we can easily write down the first 10 odd numbers to be : 1 3 5 7 9 11 13 15 17 19 .. So their sum = 10/5 x (A+L) = 10/5 x 20 = 40 ... Divide 2550 by 40 to get the desired ratio...
Hope this helps
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