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11.5: Radical Equations

Difficulty Level: Basic Created by: CK-12
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Suppose your teacher has instructed the members of your math class to work in pairs, and she has asked you to find the length of a line segment. You get 2x+6 units for the length, and your partner gets 10 units. You ask your teacher who is right, and she says both of you are right! Can you set up an equation and solve for x in this case? How would you do it? In this Concept, you'll learn to solve radical equations like the one you would set up here.


Solving radical equations is no different from solving linear or quadratic equations. Before you can begin to solve a radical equation, you must know how to cancel the radical. To do that, you must know its inverse.

Original Operation Inverse Operation
Cube Root Cubing (to the third power)
Square Root Squaring (to the second power)
Fourth Root Fourth power
nth” Root nth” power

To solve a radical equation, you apply the solving equation steps you learned in previous Concepts, including the inverse operations for roots.

Example A

Solve 2x1=5.


The first operation that must be removed is the square root. Square both sides.


Remember to check your answer by substituting it into the original problem to see if it makes sense.

Extraneous Solutions

Not every solution of a radical equation will check in the original problem. This is called an extraneous solution. This means you can find a solution using algebra, but it will not work when checked. This is because of the rule in a previous Concept:

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

or, in words, even roots of negative numbers are undefined.

Example B

Solve x3x=1.


Isolate one of the radical expressions. x3Square both sides.(x3)2Remove parentheses. x3Simplify. x3Now isolate the remaining radical.4Divide all terms by 2.2Square both sides. x=x+1=(x+1)2=(x)2+2x+1=x+2x+1=2x=x=4

Check: 43=?4+11=?2+113. The solution does not check out. The equation has no real solutions. Therefore, x=4 is an extraneous solution.

Radical Equations in Real Life

Example C

A sphere has a volume of 456 cm3. If the radius of the sphere is increased by 2 cm, what is the new volume of the sphere?


  1. Define variables. Let R= the radius of the sphere.
  2. Find an equation. The volume of a sphere is given by the formula: V=43πr3.

By substituting 456 for the volume variable, the equation becomes 456=43πr3.

Multiply by 3:1368Divide by 4π:108.92Take the cube root of each side: rThe new radius is 2 centimeters more: rThe new volume is:V=4πr3=r3=108.923r=4.776 cm=6.776 cm=43π(6.776)3=1302.5 cm3

Check by substituting the values of the radii into the volume formula.

V=43πr3=43π(4.776)3=456 cm3. The solution checks out.

Video Review

Guided Practice

Solve x+15=3x3.


Begin by canceling the square roots by squaring both sides.

(x+15)2x+15Isolate the xvariable:18x=(3x3)2=3x3=2x=9

Check the solution: 9+15=3(9)324=24. The solution checks.


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: Extraneous Solutions to Radical Equations (11:10)

CK-12 Basic Algebra: Radical Equation Examples (5:16)

CK-12 Basic Algebra: More Involved Radical Equation Example (11:54)

In 1-16, find the solution to each of the following radical equations. Identify extraneous solutions.

  1. x+22=0
  2. 3x1=5
  3. 243x+3=0
  4. x33=1
  5. x294=2
  6. 25x3+3=0
  7. x=x6
  8. x25x6=0
  9. (x+1)(x3)=x
  10. x+6=x+4
  11. x=x9+1
  12. 3x+4=6
  13. 105x+1x=7
  14. 2x22x+2=0
  15. 2x+532x3=2x
  16. 3x9=2x14
  17. The area of a triangle is 24 in2 and the height of the triangle is twice as long and the base. What are the base and the height of the triangle?
  18. The volume of a square pyramid is given by the formula V=A(h)3, where A= area of the base and h= height of the pyramid. The volume of a square pyramid is 1,600 cubic meters. If its height is 10 meters, find the area of its base.
  19. The volume of a cylinder is 245 cm3 and the height of the cylinder is one-third the diameter of the cylinder's base. The diameter of the cylinder is kept the same, but the height of the cylinder is increased by two centimeters. What is the volume of the new cylinder? (Volume=πr2h)
  20. The height of a golf ball as it travels through the air is given by the equation h=16t2+256. Find the time when the ball is at a height of 120 feet.

Mixed Review

  1. Joy sells two types of yarn: wool and synthetic. Wool is $12 per skein and synthetic is $9 per skein. If Joy sold 16 skeins of synthetic and collected a total of $432, how many skeins of wool did she sell?
  2. Solve 16|x4|.
  3. Graph the solution: {y2x4y>14x+6.
  4. You randomly point to a day in the month of February, 2011. What is the probability your finger lands on a Monday?
  5. Carbon-14 has a half life of 5,730 years. Your dog dug a bone from your yard. It had 93% of its carbon-14 remaining. How old is the bone?
  6. What is true about solutions to inconsistent systems?

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Extraneous Solution

An extraneous solution is a solution of a simplified version of an original equation that, when checked in the original equation, is not actually a solution.

Quadratic Equation

A quadratic equation is an equation that can be written in the form =ax^2 + bx + c = 0, where a, b, and c are real constants and a\ne 0.

Quadratic Formula

The quadratic formula states that for any quadratic equation in the form ax^2+bx+c=0, x=\frac{-b \pm \sqrt{b^2-4ac}}{2a}.

Radical Expression

A radical expression is an expression with numbers, operations and radicals in it.

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Difficulty Level:
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Date Created:
Feb 24, 2012
Last Modified:
Apr 11, 2016
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