# Lesson Explainer: Evaluating Series Algebraically Mathematics

In this explainer, we will learn how to evaluate linear and quadratic series by applying algebraic methods and formulas.

We will begin by recalling what we mean by a series as the sum of terms in a sequence and how we use sigma notation to represent this.

### Definition: Series Using Sigma Notation

We can take the sum of a term , which is a function of , from to some value using the sigma notation denoted by :

The series can also start from a different value, from for to some value , which can be written as which is the difference of two series with a starting index .

In order to evaluate these series algebraically, we will make use of some properties using the notation. These are intuitive properties which are reminiscent of how we work with algebraic expressions, but it is useful to familiarize ourselves with the more formal notation.

### Definition: Properties of Series

The sums of the series written in terms of the sigma notation satisfy the following properties:

• If we have a constant appearing inside the sum, we can take it out of the sum as follows:
• We can also split up the sum for different terms in the summand as follows:
• We can also split up the summation for some value in between the start and end, . If we start at and sum up to , then for we have

The first two properties show that the summation is linear and we can combine them as

For the last property, we can also see this by using the property for a starting index greater than 1, as follows:

In this explainer, we will look at linear or quadratic series, that is, where is a linear or quadratic function of .

Let’s consider an example where we use the last property to rewrite a summation.

.

### Answer

In this example, we will simplify a series algebraically.

We will make use of the property for starting indices greater than 1:

For the given summation, we have

This makes sense, since the first term is the sum from to 12, while the second sum is from to 25; thus, the total sum will be from to 25. If we apply the property where we can split the series for some as we obtain the same result upon substituting and , which gives the given series on the right-hand side and the result on the left-hand side.

This is option D.

Now let’s calculate the summation of a series with a constant inside the summand, which we can take outside the sum:

Note that

Thus for , the summation for a constant series, we have

Now, let’s look at an example where we evaluate an arithmetic series with a constant summand.

Evaluate .

### Answer

In this example, we want to evaluate an arthmetic series with a constant summand.

We will make use of the following summation:

Thus, for the given sum, we have and and hence

Now, let’s consider a linear summation of the form

By the property for adding series and the constant multiple property, we can rewrite this as

In order to evaluate this sum, we first need a formula for the series .

We can write this series out explicitly in two different ways: by starting from 1 and summing all the numbers up to and by starting from and summing all numbers down to 1:

Adding these together gives

Thus, we have

We can also derive this from the binomial expansion of , which upon rearranging gives

First note that the summation of the left-hand side from to gives while the right-hand side gives

Thus, we obtain

This gives the same formula as before:

Using this result, we can now evaluate the series of the linear expression defined by :

In the next example, we will look at evaluating a sum algebraically using the properties of summation for a series with a linear summand.

Find given .

### Answer

In this example, we will evaluate a linear series algebraically using properties of summation.

We will make use of the summation linearity property and the summations

The given summation can be written as

Now, let’s look at an example where we determine the value of an unknown appearing in a linear series with a given value for the sum.

If , then .

1. 20

### Answer

In this example, we want to find the value of an unknown appearing inside an arithmetic sum.

We will make use of the summation linearity property and the summations

Let’s first evaluate the right-hand side of the given sum:

Using this, we can determine the value of as

Now, let’s evaluate a linear series with a starting index greater than 1.

### Example 5: Evaluating Arithmetic Series with a Starting Index Greater Than 1

Find using the properties of summation and given .

### Answer

In this example, we will evaluate a linear arithmetic series with a starting index greater than 1 using the properties of summation.

We will make use of the property for starting indices greater than 1 and the summation linearity property and the summations

Using the properties, we can rewrite the given sum as

Now, let’s consider a quadratic expression where we want to evaluate the series:

By the property for adding series and the constant multiple property, we can rewrite this as

In order to evaluate this sum, we first need a formula for the series .

Similar to the linear term, we will consider the binomial expansion of , which upon rearranging can be written as

The summation of the left-hand side from to gives and from the right-hand side we have

Thus, we have and upon rearranging we obtain the formula

Using this result, we can evaluate the series as follows:

Let’s look at a few examples of how to evaluate a quadratic series. In the next example, we will evaluate a summand that contains a square and constant term using the properties of summation.

### Example 6: Evaluating the Sum of a Finite Quadratic Series Using the Properties of Summation

Given , use the properties of summation to find .

### Answer

In this example, we will evaluate a quadratic series containing a square and constant term, using the properties of summation.

We will make use of the summation linearity property and the summations

Using these properties and summations, we can evaluate the given series:

Now, let’s look at an example where we will evaluate a summand that contains a quadratic function of .

### Example 7: Evaluating the Sum of a Finite Quadratic Series Using the Properties of Summation

Given that and , use the properties of the summation notation to find .

### Answer

In this example, we will evaluate a quadratic series using the properties of summation.

We will make use of the summation linearity property and the summations

Using these properties and summations, we can evaluate the given series:

Finally, let’s evaluate a quadratic series with a starting index greater than 1.

### Example 8: Evaluating the Sum of a Finite Quadratic Series with a Starting Index Greater Than 1 Using the Properties of Summation

Given that and , use the properties of the summation notation to find .

### Answer

We will make use of the property for starting indices greater than 1 and the summation linearity property and the summations

Using these properties and summations, we can evaluate the given series:

### Key Points

• We have evaluated different series of the form where for a linear series, while for a quadratic series.
• In order to evaluate these, we make use of the properties The first allows us to evaluate a series with a starting index greater than one, by writing it as a difference of two series of a starting index equal to one. The second is a linearity property, which allows us to split the summation for different terms and take out the constant.
• We also make use of the following series:
• When the starting index is equal to 1, that is, , we have the following result for the general quadratic series:

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