**Elementary School Number Sense Lesson 25: Adding a Sequence of Consecutive Integers (Starting with 1)**

This concept is closely related to

**AddSeqEven**and

**AddSeqOdd**. It appeared

**3 times**this year, with a median placement at

**question # 4**.

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For this method to work, the sequence needs to:

- Start with 1, and
- Include consecutive integers without skipping

**Use this formula:**

**n(n+1)/2**,

*nth Triangular Number*(

**TriangularNum**).

**: Make sure to divide the even number by 2 before you multiply.**

__Note__**Example 1: 1 + 2 + 3 + 4 + 5 + 6 = ___**

- Use
**n(n+1)/2**. - 6(6 + 1)/2 = 6(7)/2 = 3(7) =
**21**

**Example 2: 1 + 2 + 3 + … + 11 = ___**

- Use
**n(n+1)/2**. - 11(11 + 1)/2 = 11(12)/2 = 11(6) =
**66**

**Example 3: 1 + 2 + 3 + … + 14 = ___**

- Use
**n(n+1)/2**. - 14(14 + 1)/2 = 14(15)/2 = 7(15) =
**105**

**Example 4: 1 + 2 + 3 + 4 + … + 21 = ___**

- Use
**n(n+1)/2**. - 21(21 + 1)/2 = 21(22)/2 = 21(11) =
**231**

**Example 5: 1 + 2 + 3 + 4 + … + 24 = ___**

- Use
**n(n+1)/2**. - 24(24 + 1)/2 = 24(25)/2 = 12(25) =
**300**

**Example 6: 1 + 2 + 3 + 4 + … + 49 = ___**

- Use
**n(n+1)/2**. - 49(49 + 1)/2 = 49(50)/2 = 49(25) =
**1225**

**Up Next for Elementary School: Mult15**