What is the sum of odd numbers from 4 to 21? Here we will show you step-by-step how to calculate the sum of odd numbers from 4 to 21.
Step 1) We start by finding the first (F) and last (L) odd numbers from 4 to 21. It is easy to do that by simply looking at the numbers. The first odd number is 5, and the last odd number is 21.
F = 5
L = 21
Step 2) Next, we calculate the total odd numbers (T) there are from 4 to 21, using a formula. Here is the formula and the math:
T = (L - F + 2) ÷ 2
T = (21 - 5 + 2) ÷ 2
T = 18 ÷ 2
T = 9
Step 3) Finally, we use the information from Step 1 and Step 2, and another formula, to calculate the sum of odd numbers from 4 to 21, like this:
Sum = (T ÷ 2) × (2 × F + (2 × (T - 1))
Sum = (9 ÷ 2) × (2 × 5 + (2 × (9 - 1))
Sum = 4.5 × (10 + 16)
Sum = 4.5 × 26
Sum = 117
Sum of odd numbers from 4 to 21 = 117
Sum of Odd Numbers Calculator
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