What is the sum of even numbers from 9 to 21? Here we will show you step-by-step how to calculate the sum of even numbers from 9 to 21.
Step 1) We start by finding the first (F) and last (L) even numbers from 9 to 21. It is easy to do that by simply looking at the numbers. The first even number is 10, and the last even number is 20.
F = 10
L = 20
Step 2) Next, we calculate the total even numbers (T) there are from 9 to 21, using a formula. Here is the formula and the math:
T = (L - F + 2) ÷ 2
T = (20 - 10 + 2) ÷ 2
T = 12 ÷ 2
T = 6
Step 3) Finally, we use the information from Step 1 and Step 2, and another formula, to calculate the sum of even numbers from 9 to 21, like this:
Sum = (T ÷ 2) × (2 × F + (2 × (T - 1))
Sum = (6 ÷ 2) × (2 × 10 + (2 × (6 - 1))
Sum = 3 × (20 + 10)
Sum = 3 × 30
Sum = 90
Sum of even numbers from 9 to 21 = 90
Sum of Even Numbers Calculator
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