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9.2: Addition and Subtraction of Polynomials

Difficulty Level: Basic Created by: CK-12
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Suppose that two cars are having a race. The distance traveled by one car after \begin{align*}t\end{align*} seconds is \begin{align*}10t^2 + 50t\end{align*} meters, while the distance traveled by the other car after \begin{align*}t\end{align*} seconds is \begin{align*}15t^2 + 40t\end{align*} meters. How far would the two cars be apart after \begin{align*}t\end{align*} seconds? What would you have to do to find the answer to this question? In this Concept, you'll learn how to add and subtract polynomials so that you can solve problems like this one.

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Multimedia Link: For more explanation of polynomials, visit http://www.purplemath.com/modules/polydefs.htm - Purplemath’s website.


To add or subtract polynomials, you have to group the like terms together and combine them to simplify.

Example A

Add and simplify \begin{align*}3x^2-4x+7\end{align*} and \begin{align*}2x^3-4x^2-6x+5\end{align*}.

Solution: Add \begin{align*}3x^2-4x+7\end{align*} and \begin{align*}2x^3-4x^2-6x+5\end{align*}.

\begin{align*}(3x^2-4x+7)+(2x^3-4x^2-6x+5)&=2x^3+(3x^2-4x^2 )+(-4x-6x)+(7+5)\\ &=2x^3-x^2-10x+12\end{align*}

Example B

Subtract \begin{align*}5b^2-2a^2\end{align*} from \begin{align*}4a^2-8ab-9b^2\end{align*}.


\begin{align*}(4a^2-8ab-9b^2)-(5b^2-2a^2)&=[(4a^2- (-2a^2)]+(-9b^2-5b^2)-8ab\\ & = 6a^2-14b^2-8ab\end{align*}

Solving Real-World Problems Using Addition or Subtraction of Polynomials

Polynomials are useful for finding the areas of geometric objects. In the following examples, you will see this usefulness in action.

Example C

Write a polynomial that represents the area of each figure shown.



Solution: The blue square has the following area: \begin{align*}y \cdot y=y^2\end{align*}.

The yellow square has the following area: \begin{align*}x \cdot x = x^2\end{align*}.

The pink rectangles each have the following area: \begin{align*}x \cdot y =xy\end{align*}.

\begin{align*}\text{Test area} & = y^2+x^2+xy+xy\\ & = y^2 + x^2 + 2xy\end{align*}

To find the area of the green region we find the area of the big square and subtract the area of the little square.

The big square has an area of \begin{align*}y \cdot y =y^2\end{align*}.

The little square has an area of \begin{align*}x \cdot x = x^2\end{align*}.

Area of the green region \begin{align*}= y^2-x^2\end{align*}

Guided Practice

Subtract \begin{align*} 4t^2+7t^3-3t-5\end{align*} from \begin{align*}6t+3-5t^3+9t^2\end{align*}.


When subtracting polynomials, we have to remember to subtract each term. If the term is already negative, subtracting a negative term is the same thing as adding:

\begin{align*}6t+3-5t^3+9t^2-(4t^2+7t^3-3t-5)&= \\ 6t+3-5t^3+9t^2-(4t^2)-(7t^3)-(-3t)-(-5)&=\\ 6t+3-5t^3+9t^2-4t^2-7t^3+3t+5&=\\ (6t+3t)+(3+5)+(-5t^3-7t^3)+(9t^2-4t^2)&=\\ 9t+8-12t^3+5t^2&=\\ -12t^3+5t^2+9t+8\\ \end{align*}

The final answer is in standard form.


Sample explanations for some of the practice exercises below are available by viewing the following video. Note that there is not always a match between the number of the practice exercise in the video and the number of the practice exercise listed in the following exercise set. However, the practice exercise is the same in both. CK-12 Basic Algebra: Addition and Subtraction of Polynomials (15:59)

Add and simplify.

  1. \begin{align*}(x+8)+(-3x-5)\end{align*}
  2. \begin{align*}(8r^4-6r^2-3r+9)+(3r^3+5r^2+12r-9)\end{align*}
  3. \begin{align*}(-2x^2+4x-12) + (7x+x^2)\end{align*}
  4. \begin{align*}(2a^2b-2a+9)+(5a^2b-4b+5)\end{align*}
  5. \begin{align*}(6.9a^2-2.3b^2+2ab)+(3.1a-2.5b^2+b)\end{align*}

Subtract and simplify.

  1. \begin{align*}(-t+15t^2)-(5t^2+2t-9)\end{align*}
  2. \begin{align*}(-y^2+4y-5)-(5y^2+2y+7)\end{align*}
  3. \begin{align*}(-h^7+2h^5+13h^3+4h^2-h-1)-(-3h^5+20h^3-3h^2+8h-4)\end{align*}
  4. \begin{align*}(-5m^2-m)-(3m^2+4m-5)\end{align*}
  5. \begin{align*}(2a^2b-3ab^2+5a^2b^2)-(2a^2b^2+4a^2b-5b^2)\end{align*}

Find the area of the following figures.

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constant In a polynomial, the number appearing all by itself without a variable is called the constant.
like Terms Like terms are terms in the polynomial that have the same variable(s) with the same exponents, but they can have different coefficients.
Polynomial A polynomial is an expression with at least one algebraic term, but which does not indicate division by a variable or contain variables with fractional exponents.
distributive property The distributive property states that the product of an expression and a sum is equal to the sum of the products of the expression and each term in the sum. For example, a(b + c) = ab + ac.

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