Mathematics
Grade 11
15 min
Classify formulas and sequences
Classify formulas and sequences
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1
Introduction & Learning Objectives
Learning Objectives
Distinguish between arithmetic and geometric sequences by analyzing their common difference or common ratio.
Identify a sequence as neither arithmetic nor geometric if it lacks a constant difference or ratio.
Classify a formula for a sequence as either explicit or recursive.
Write the first few terms of a sequence given its explicit formula.
Write the first few terms of a sequence given its recursive formula and initial term(s).
Determine the type of sequence (arithmetic, geometric, or neither) from a given list of terms.
Ever notice how a sunflower's seeds spiral outwards or how your savings grow with interest? 🌱 These are real-life sequences! But how do we describe their patterns mathematically?
In this tutorial, we will learn how to identify and...
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Key Concepts & Vocabulary
TermDefinitionExample
SequenceAn ordered list of numbers, called terms, that follow a specific pattern or rule.The list 2, 4, 6, 8, 10, ... is a sequence.
Arithmetic SequenceA sequence where the difference between any two consecutive terms is constant. This constant value is called the common difference (d).In the sequence 5, 8, 11, 14, ..., the common difference is d = 3.
Geometric SequenceA sequence where the ratio of any two consecutive terms is constant. This constant value is called the common ratio (r).In the sequence 2, 6, 18, 54, ..., the common ratio is r = 3.
Explicit FormulaA formula that defines the nth term of a sequence, a_n, as a function of its position, n. It allows for the direct calculation of any term in the sequence.a_n = 2n + 1. To find the 10th term, you just plug i...
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Core Formulas
Explicit Formula for an Arithmetic Sequence
a_n = a_1 + (n-1)d
Use this to find any term (a_n) in an arithmetic sequence when you know the first term (a_1) and the common difference (d).
Recursive Formula for an Arithmetic Sequence
a_n = a_{n-1} + d, given a_1
Use this to define a term based on the previous term. You must state the first term, a_1, to provide a starting point.
Explicit Formula for a Geometric Sequence
a_n = a_1 * r^(n-1)
Use this to find any term (a_n) in a geometric sequence when you know the first term (a_1) and the common ratio (r).
Recursive Formula for a Geometric Sequence
a_n = a_{n-1} * r, given a_1
Use this to define a term based on the previous term. You must state the first term, a_1, to provide a starting point.
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Challenging
Which of the following formulas correctly defines the sequence 4, 12, 36, 108, ...?
A.a_n = 4 + (n-1)8
B.a_n = a_{n-1} + 8, with a_1 = 4
C.a_n = 4 * 3^(n-1)
D.a_n = 4 * n^3
Challenging
A sequence is defined by the recursive formula `a_n = 2 * a_{n-1} - a_{n-2}` with `a_1 = 5` and `a_2 = 8`. What is the 4th term, `a_4`, and how is the sequence classified?
A.a_4 = 14, Arithmetic
B.a_4 = 16, Geometric
C.a_4 = 11, Neither
D.a_4 = 14, Neither
Challenging
A population of algae starts at 100 cells and triples every day. You need a formula to find the population on any given day `n` without needing to know the population from the day before. Which description is most accurate?
A.An explicit formula for an arithmetic sequence.
B.recursive formula for an arithmetic sequence.
C.An explicit formula for a geometric sequence.
D.recursive formula for a geometric sequence.
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