Question

1+3+5+7+9+11+13+15+17+19+21 without adding

Answer 1

121

Explanation:

1 + 3 + 5 + 7 + 9 + 11 + 13 + 15 + 17 + 19 + 21 is a sum of first 11 odd numbers.

`\sf \boxed{\text{\bf Sum of first n odd numbers = $n^2$}}`

Here, n is the number of terms.

In this sequence, there are 11 terms.

Sum of first 11 odd numbers = 11²

= 11 *11

= 121

Answer 2

121

Explanation:

It's a series of odd numbers. Let

`S _(11) = 1+3+5+7+9+11+13+15+17+19+21`

As a rule of thumb, the sum of n numbers of odd numbers is equal to n². There're 11 numbers in this series. Hence The sum is 11² =121