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11.2: Radical Expressions

Difficulty Level: At Grade Created by: CK-12

Radicals are the roots of values. In fact, the word radical comes from the Latin word “radix,” meaning “root.” You are most comfortable with the square root symbol x; however, there are many more radical symbols.

A radical is a mathematical expression involving a root by means of a radical sign.

y3=xy4=xyn=xbecause x3=ybecause x4=ybecause xn=y273=3, because 33=27164=2 because 24=16

Some roots do not have real values; in this case, they are called undefined.

Even roots of negative numbers are undefined.

xn is undefined when n is an even whole number and x<0.

Example 1: Evaluate the following radicals:

  • 643
  • 814

Solution: 643=4 because 43=64

814 is undefined because n is an even whole number and 81<0.

In Chapter 8, you learned how to evaluate rational exponents:

axy where x=power and y=root

This can be written in radical notation using the following property.

Rational Exponent Property: For integer values of x and whole values of y:

axy=axy

Example: Rewrite x56 using radical notation.

Solution: This is correctly read as the sixth root of x to the fifth power. Writing in radical notation, x56=x56, where x5>0.

Example 2: Evaluate 424.

Solution: This is read, “The fourth root of four to the second power.”

42=16

The fourth root of 16 is 2; therefore,

424=2

In Chapter 1, Lesson 5, you learned how to simplify a square root. You can also simplify other radicals, like cube roots and fourth roots.

Example: Simplify 1353.

Solution: Begin by finding the prime factorization of 135. This is easily done by using a factor tree.

1353=33353=33353353

Adding or Subtracting Radicals

To add or subtract radicals, they must have the same root and radicand.

axn+bxn=(a+b)xn

Example 3: Add 35+65.

Solution: The value “5” is considered a like term. Using the rule above,

35+65=(3+6)5=95

Example: Simplify 2133+6123.

Solution: The cube roots are not like terms, therefore there can be no further simplification.

In some cases, the radical may need to be reduced before addition/subtraction is possible.

Example 4: Simplify 43+212.

Solution: 12 simplifies as 23.

43+21243+4343+2(23)=83

Multiplying or Dividing Radicals

To multiply radicands, the roots must be the same.

anbn=abn

Example: Simplify 312.

Solution: 312=36=6

Dividing radicals is more complicated. A radical in the denominator of a fraction is not considered simplified by mathematicians. In order to simplify the fraction, you must rationalize the denominator.

To rationalize the denominator means to remove any radical signs from the denominator of the fraction using multiplication.

Remember:

a×a=a2=a

Example 1: Simplify 23.

Solution: We must clear the denominator of its radical using the property above. Remember, what you do to one piece of a fraction, you must do to all pieces of the fraction.

23×33=2332=233

Example: Simplify 753.

Solution: In this case, we need to make the number inside the cube root a perfect cube. We need to multiply the numerator and the denominator by 523.

753523523=7325533=73255

Real-World Radicals

Example: A pool is twice as long as it is wide and is surrounded by a walkway of uniform width of 1 foot. The combined area of the pool and the walkway is 400 square-feet. Find the dimensions of the pool and the area of the pool.

Solution:

  1. Make a sketch.
  2. Let x= the width of the pool.
  3. Write an equation. Area=lengthwidth

Combined length of pool and walkway =2x+2

Combined width of pool and walkway =x+2

Area=(2x+2)(x+2)

Since the combined area of pool and walkway is 400 ft2 we can write the equation.

(2x+2)(x+2)=400

4. Solve the equation:

Multiply in order to eliminate the parentheses.Collect like terms.Move all terms to one side of the equation.Divide all terms by 2.(2x+2)(x+2)=4002x2+4x+2x+4=4002x2+6x+4=4002x2+6x396=0x2+3x198=0

x=b±b24ac2a=3±324(1)(198)2(1)=3±80123±28.32

Use the Quadratic Formula. x12.65 or –15.65 feet

5. We can disregard the negative solution since it does not make sense for this context. Thus, we can check our answer of 12.65 by substituting the result in the area formula.

Area=[2(12.65)+2)](12.65+2)=27.314.65400 ft2.

The answer checks out.

Practice Set

Sample explanations for some of the practice exercises below are available by viewing the following videos. Note that there is not always a match between the number of the practice exercise in the videos 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: Radical Expressions with Higher Roots (8:46)

CK-12 Basic Algebra: More Simplifying Radical Expressions (7:57)

CK-12 Basic Algebra: How to Rationalize a Denominator (10:18)

  1. For which values of n is 16n undefined?

Evaluate each radical expression.

  1. 169
  2. 814
  3. 1253
  4. 10245

Write each expression as a rational exponent.

  1. 143
  2. zw4
  3. a
  4. y39

Write the following expressions in simplest radical form.

  1. 24
  2. 300
  3. 965
  4. 240567
  5. 5003
  6. 64x86
  7. 48a3b73
  8. 16x5135y43
  9. True or false? 5766=3042

Simplify the following expressions as much as possible.

  1. 38632
  2. 180+6405
  3. 627+254+348
  4. 8x34x98x
  5. 48a+27a
  6. 4x33+x2563

Multiply the following expressions.

  1. 6(10+8)
  2. (ab)(a+b)
  3. (2x+5)(2x+5)

Rationalize the denominator.

  1. 715
  2. 910
  3. 2x5x
  4. 53y
  5. The volume of a spherical balloon is 950cm3. Find the radius of the balloon. (Volume of a sphere =43πR3)
  6. A rectangular picture is 9 inches wide and 12 inches long. The picture has a frame of uniform width. If the combined area of picture and frame is 180in2, what is the width of the frame?
  7. The volume of a soda can is 355 cm3. The height of the can is four times the radius of the base. Find the radius of the base of the cylinder.

Mixed Review

  1. An item originally priced $c is marked down 15%. The new price is $612.99. What is c?
  2. Solve x+36=21x.
  3. According to the Economic Policy Institute (EPI), minimum wage in 1989 was $3.35 per hour. In 2009, it was $7.25 per hour. What is the average rate of change?
  4. What is the vertex of y=2(x+1)2+4? Is this a minimum or a maximum?
  5. Using the minimum wage data (adjusted for inflation) compiled from EPI, answer the following questions.
    1. Graph the data as a scatter plot.
    2. Which is the best model for this data: linear, quadratic, or exponential?
    3. Find the model of best fit and use it to predict minimum wage adjusted for inflation for 1999.
    4. According to EPI, the 1999 minimum wage adjusted for inflation was $6.58. How close was your model?
    5. Use interpolation to find minimum wage in 1962.
Year Minimum Wage Adj. for Inflation Year Minimum Wage Adj. for Inflation
1947 3.40 1952 5.36
1957 6.74 1960 6.40
1965 7.52 1970 7.81
1978 7.93 1981 7.52
1986 6.21 1990 6.00
1993 6.16 1997 6.81
2000 6.37 2004 5.80
2006 5.44 2008 6.48
2009 7.25

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