Question : Find the average of odd numbers from 9 to 729
Correct Answer 369
Solution & Explanation
Solution
Method (1) to find the average of the odd numbers from 9 to 729
Shortcut Trick to find the average of the given continuous odd numbers
The odd numbers from 9 to 729 are
9, 11, 13, . . . . 729
After observing the above list of the odd numbers from 9 to 729 we find that the difference between two consecutive terms are equal. This means the list of the odd numbers from 9 to 729 form an Arithmetic Series.
In the Arithmetic Series of the odd numbers from 9 to 729
The First Term (a) = 9
The Common Difference (d) = 2
And the last term (ℓ) = 729
The average of the numbers forming an Arithmetic Series
= The first term (a) + The last term (ℓ)/2
⇒ The average of numbers forming an Arithmetic Series = a + ℓ/2
Thus, the average of the odd numbers from 9 to 729
= 9 + 729/2
= 738/2 = 369
Thus, the average of the odd numbers from 9 to 729 = 369 Answer
Method (2) to find the average of the odd numbers from 9 to 729
Finding the average of given continuous odd numbers after finding their sum
The odd numbers from 9 to 729 are
9, 11, 13, . . . . 729
The odd numbers from 9 to 729 form an Arithmetic Series in which
The First Term (a) = 9
The Common Difference (d) = 2
And the last term (ℓ) = 729
The Average of the given numbers
= Sum of the given numbers/Total number of given numbers
Thus, to find the average of the given numbers, first, we need to find their sum and the total number of given numbers
Finding the number of terms
For an Arithmetic Series, the nth term
an = a + (n – 1) d
Where
a = First term
d = Common difference
n = number of terms
an = nth term
Thus, for the given series of the odd numbers from 9 to 729
729 = 9 + (n – 1) × 2
⇒ 729 = 9 + 2 n – 2
⇒ 729 = 9 – 2 + 2 n
⇒ 729 = 7 + 2 n
After transposing 7 to LHS
⇒ 729 – 7 = 2 n
⇒ 722 = 2 n
After rearranging the above expression
⇒ 2 n = 722
After transposing 2 to RHS
⇒ n = 722/2
⇒ n = 361
Thus, the number of terms of odd numbers from 9 to 729 = 361
This means 729 is the 361th term.
Finding the sum of the given odd numbers from 9 to 729
The sum of all terms (S) in an Arithmetic Series
= n/2 (a + ℓ)
Where, n = number of terms
a = First term
And, ℓ = Last term
Thus, the sum of all terms (S) of the given odd numbers from 9 to 729
= 361/2 (9 + 729)
= 361/2 × 738
= 361 × 738/2
= 266418/2 = 133209
Thus, the sum of all terms of the given odd numbers from 9 to 729 = 133209
And, the total number of terms = 361
Since, the average of the given numbers
= Sum of the given numbers/Total number of given numbers
Thus, the average of the given odd numbers from 9 to 729
= 133209/361 = 369
Thus, the average of the given odd numbers from 9 to 729 = 369 Answer
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