等差数列

The common difference is the fixed constant value added to or subtracted from each term to get the next term in an arithmetic sequence. In the pole pile example, the common difference is -1 because the number of poles decreases by one in each successive layer. This consistent pattern allows you to predict any term in the sequence without listing all preceding terms.

Use the formula aₙ = a + (n - 1)d, where aₙ is the nth term, a is the first term, d is the common difference, and n is the term position. For the pole pile with 25 poles in layer 1 and common difference -1, the 12th layer contains 25 + (12 - 1)(-1) = 14 poles. This formula eliminates the need to calculate every preceding term.

The partial sum Sₙ equals n times the average of the first and last terms: Sₙ = n(a + aₙ)/2. For the 12-layer pole pile, the total is 12 × (25 + 14)/2 = 234 poles. This formula efficiently computes the sum of the first n terms without adding each term individually, making it practical for large sequences.

The pole pile forms an arithmetic sequence because each layer contains a constant decrease of one pole from the previous layer. This consistent pattern of change—the common difference—is the defining characteristic of arithmetic sequences. The structured arrangement allows systematic calculation of any layer's pole count and the total across all layers.

A partial sum calculates only the first n terms of a sequence, as shown when finding the total of 12 layers in the pole pile. The partial sum formula Sₙ = n(a + aₙ)/2 applies to a finite number of terms. This approach is practical for real-world applications where you need the cumulative total of a specific portion rather than an entire infinite sequence.

Yes, arithmetic sequences model real-world stacking scenarios like the pole pile, where items decrease or increase by a constant amount per layer. By identifying the first term, common difference, and number of layers, you can calculate individual layer quantities using the nth term formula and total inventory using summation notation. This systematic approach simplifies inventory and structural calculations.

Arithmetic sequences use addition of a constant common difference, while geometric sequences use multiplication by a constant ratio. Both are structured numerical patterns that allow efficient term and sum calculations. Understanding arithmetic sequences provides foundational knowledge for comparing with geometric sequences and recognizing which pattern applies to different real-world or theoretical problems.