The sum of the first n terms of an arithmetic sequence is 153, with first term 5 and common difference 3. Find n. - Parker Core Knowledge

Understanding the Context

Why This Calculation Is Gaining Attention

In recent years, linear learning—particularly through educational apps and social platforms—has sparked curiosity in how simple math models real-life scenarios. The arithmetic sequence offers a clean, predictable pattern: start at 5, rise by 3 each step. The idea that such a structure fits a total of 153 invites deeper exploration. Users searching online aren’t news-seekers—they’re learners, students, educators, and professionals using math to understand trends, budgets, or growth models. This problem reflects a broader trend: people seeking clarity in numbers behind everyday experiences. Platforms that deliver clear, factual explanations earn trust and visibility, especially on mobile where curious users scroll quickly but linger on trusted insights.

How It Actually Works: A Clear Explanation

An arithmetic sequence follows a fixed pattern: each term increases by a constant difference—in this case, 3. The general formula for the sum of the first n terms is:

Key Insights

Sₙ = n/2 × (2a + (n−1)d)

Where:
Sₙ = sum of the first n terms
a = first term = 5
d = common difference = 3
n = number of terms (what we’re solving for)

Plugging in known values:

153 = n/2 × (2×5 + (n−1)×3)
153 = n/2 × (10 + 3n − 3)
153 = n/2 × (3n + 7)

Multiply both sides by 2:

Final Thoughts

306 = n(3n + 7)
306 = 3n² + 7n

Rearranging into standard quadratic form:

3n² + 7n — 306 = 0

Apply the quadratic formula: n = [-b ± √(b² − 4ac)] / 2a
With a = 3, b = 7, c = -306:

Discriminant: b² − 4ac = 49 + 3672 = 3721
√3721 = 61

n = [-7 ± 61] / 6