What is the sum of odd numbers from 21 to 91? Here we will show you step-by-step how to calculate the sum of odd numbers from 21 to 91.
Step 1) We start by finding the first (F) and last (L) odd numbers from 21 to 91. It is easy to do that by simply looking at the numbers. The first odd number is 21, and the last odd number is 91.
F = 21
L = 91
Step 2) Next, we calculate the total odd numbers (T) there are from 21 to 91, using a formula. Here is the formula and the math:
T = (L - F + 2) ÷ 2
T = (91 - 21 + 2) ÷ 2
T = 72 ÷ 2
T = 36
Step 3) Finally, we use the information from Step 1 and Step 2, and another formula, to calculate the sum of odd numbers from 21 to 91, like this:
Sum = (T ÷ 2) × (2 × F + (2 × (T - 1))
Sum = (36 ÷ 2) × (2 × 21 + (2 × (36 - 1))
Sum = 18 × (42 + 70)
Sum = 18 × 112
Sum = 2016
Sum of odd numbers from 21 to 91 = 2016
Sum of Odd Numbers Calculator
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