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The radical symbol (√) represents the square root of a number. You can encounter the radical symbol in algebra or even in carpentry or another trade that involves geometry or calculating relative sizes or distances. You can multiply any two radicals that have the same indices (degrees of a root) together. If the radicals do not have the same indices, you can manipulate the equation until they do. If you want to know how to multiply radicals with or without coefficients, just follow these steps.
Steps
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Make sure that the radicals have the same index. To multiply radicals using the basic method, they have to have the same index. The "index" is the very small number written just to the left of the uppermost line in the radical symbol. If there is no index number, the radical is understood to be a square root (index 2) and can be multiplied with other square roots. You can multiply radicals with different indexes, but that is a more advanced method and will be explained later. Here are two examples of multiplication using radicals with the same indexes:
- Ex. 1 : √(18) x √(2) = ?
- Ex. 2 : √(10) x √(5) = ?
- Ex. 3 : 3 √(3) x 3 √(9) = ?
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Multiply the numbers under the radical signs. Next, simply multiply the numbers under the radical or square root signs and keep them there. Here's how you do it: [1] X Research source
- Ex. 1 : √(18) x √(2) = √(36)
- Ex. 2 : √(10) x √(5) = √(50)
- Ex. 3 : 3 √(3) x 3 √(9) = 3 √(27)
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Simplify the radical expressions. If you've multiplied radicals, there's a good chance that they can be simplified to perfect squares or perfect cubes, or that they can be simplified by finding a perfect square as a factor of the final product. Here's how you do it: [2] X Research source
- Ex. 1: √(36) = 6. 36 is a perfect square because it is the product of 6 x 6. The square root of 36 is simply 6.
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Ex. 2: √(50) = √(25 x 2) = √([5 x 5] x 2) = 5√(2). Though 50 is not a perfect square, 25 is a factor of 50 (because it divides evenly into the number) and is a perfect square. You can break 25 down into its factors, 5 x 5, and move one 5 out of the square root sign to simplify the expression.
- You can think of it like this: If you throw the 5 back under the radical, it is multiplied by itself and becomes 25 again.
- Ex. 3: 3 √(27) = 3. 27 is a perfect cube because it's the product of 3 x 3 x 3. The cube root of 27 is therefore 3.
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Multiply the coefficients. The coefficients are the numbers outside of a radical. If there is no given coefficient, then the coefficient can be understood to be 1. Multiply the coefficients together. Here's how you do it: [3] X Research source
- Ex. 1
: 3√(2) x √(10) = 3√( ? )
- 3 x 1 = 3
- Ex. 2
: 4√(3) x 3√(6) = 12√( ? )
- 4 x 3 = 12
- Ex. 1
: 3√(2) x √(10) = 3√( ? )
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Multiply the numbers inside the radicals. After you've multiplied the coefficients, you can multiply the numbers inside the radicals. Here's how you do it: [4] X Research source
- Ex. 1 : 3√(2) x √(10) = 3√(2 x 10) = 3√(20)
- Ex. 2 : 4√(3) x 3√(6) = 12√(3 x 6) = 12√(18)
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Simplify the product. Next, simplify the numbers under the radicals by looking for perfect squares or multiples of the numbers under the radicals that are perfect squares. Once you've simplified those terms, just multiply them by their corresponding coefficients. Here's how you do it: [5] X Research source
- 3√(20) = 3√(4 x 5) = 3√([2 x 2] x 5) = (3 x 2)√(5) = 6√(5)
- 12√(18) = 12√(9 x 2) = 12√(3 x 3 x 2) = (12 x 3)√(2) = 36√(2)
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Find the LCM (lowest common multiple) of the indices. To find the LCM of the indexes, find the smallest number that is evenly divisible by both indices. Find the LCM of the indices for the following equation: 3 √(5) x 2 √(2) = ? [6] X Research source
- The indices are 3 and 2. 6 is the LCM of these two numbers because it is the smallest number that is evenly divisible by both 3 and 2. 6/3 = 2 and 6/2 = 3. To multiply the radicals, both of the indices will have to be 6.
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Write each expression with the new LCM as the index. Here's what the expressions would look like in the equation with their new indexes: [7] X Research source
- 6 √(5) x 6 √(2) = ?
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Find the number that you would need to multiply each original index by to find the LCM. For the expression 3 √(5), you'd need to multiply the index of 3 by 2 to get 6. For the expression 2 √(2), you'd need to multiply the index of 2 by 3 to get 6.
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Make this number the exponent of the number inside the radical. For the first equation, make the number 2 the exponent over the number 5. For the second equation, make the number 3 the exponent over the number 2. Here's what it would look like: [8] X Research source
- 2 --> 6 √(5) = 6 √(5) 2
- 3 --> 6 √(2) = 6 √(2) 3
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Multiply the numbers inside the radicals by their exponents. Here's how you do it:
- 6 √(5) 2 = 6 √(5 x 5) = 6 √25
- 6 √(2) 3 = 6 √(2 x 2 x 2) = 6 √8
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Place these numbers under one radical. Place them under a radical and connect them with a multiplication sign. Here's what the result would look like: [9] X Research source 6 √(8 x 25)
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Multiply them. 6 √(8 x 25) = 6 √(200). This is the final answer. In some cases, you may be able to simplify these expressions -- for example, you could simplify this expression if you found a number that can be multiplied by itself six times that is a factor of 200. But in this case, the expression cannot be simplified any further. [10] X Research source
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Expert Q&A
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QuestionHow do I multiply a radical by a whole number?Jake Adams is an academic tutor and the owner of Simplifi EDU, a Santa Monica, California based online tutoring business offering learning resources and online tutors for academic subjects K-College, SAT & ACT prep, and college admissions applications. With over 14 years of professional tutoring experience, Jake is dedicated to providing his clients the very best online tutoring experience and access to a network of excellent undergraduate and graduate-level tutors from top colleges all over the nation. Jake holds a BS in International Business and Marketing from Pepperdine University.So, you have two options: simplify the radical as much as possible, and then multiply the number outside the radical by any other number in the multiplication, leaving your answer in radical form. Alternatively, if you prefer a decimal result, you can multiply the number by the decimal approximation of the radical expression. These are the two methods that come to mind because it would depend on your formula, solution, or goal.
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QuestionWhat does index of 4 mean?DonaganTop AnswererAn index of 4 means the fourth root.
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QuestionCan I multiply a number inside the radical with a number outside the radical?Community AnswerOnly if you are reversing the simplification process. For example, 3 with a radical of 8. 3 squared is 9, so you multiply 9 under the radical with the eight for the original. It would be 72 under the radical.
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Tips
- If a "coefficient" is separated from the radical sign by a plus or minus sign, it's not a coefficient at all--it's a separate term and must be handled separately from the radical. If a radical and another term are both enclosed in the same set of parentheses--for example, (2 + (square root)5), you must handle both 2 and (square root)5 separately when performing operations inside the parentheses, but when performing operations outside the parentheses you must handle (2 + (square root)5) as a single whole.Thanks
- Radical signs are another way of expressing fractional exponents. In other words, the square root of any number is the same as that number raised to the 1/2 power, the cube root of any number is the same as that number raised to the 1/3 power, and so on.Thanks
- A "coefficient" is the number, if any, placed directly in front of a radical sign. So for example, in the expression 2(square root)5, 5 is beneath the radical sign and the number 2, outside the radical, is the coefficient. When a radical and a coefficient are placed together, it's understood to mean the same thing as multiplying the radical by the coefficient, or to continue the example, 2 * (square root)5.Thanks
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References
- ↑ https://math.libretexts.org/Bookshelves/Algebra/Book%3A_Advanced_Algebra/05%3A_Radical_Functions_and_Equations/5.04%3A_Multiplying_and_Dividing_Radical_Expressions
- ↑ https://www.chilimath.com/lessons/intermediate-algebra/multiplying-radical-expressions/
- ↑ https://www.youtube.com/watch?v=oPA8h7eccT8
- ↑ https://www.purplemath.com/modules/radicals2.htm
- ↑ https://www.themathpage.com/alg/multiply-radicals.htm
- ↑ https://www.youtube.com/watch?v=xCKvGW_39ws
- ↑ https://math.libretexts.org/Bookshelves/Algebra/Intermediate_Algebra_for_Science_Technology_Engineering_and_Mathematics_(Diaz)/10%3A_Radicals/10.05%3A_Radicals_with_Mixed_Indices
- ↑ https://math.libretexts.org/Bookshelves/Algebra/Intermediate_Algebra_for_Science_Technology_Engineering_and_Mathematics_(Diaz)/10%3A_Radicals/10.05%3A_Radicals_with_Mixed_Indices
- ↑ https://math.libretexts.org/Bookshelves/Algebra/Intermediate_Algebra_for_Science_Technology_Engineering_and_Mathematics_(Diaz)/10%3A_Radicals/10.05%3A_Radicals_with_Mixed_Indices
About This Article
Article Summary
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To multiply radicals, first verify that the radicals have the same index, which is the small number to the left of the top line in the radical symbol. Just keep in mind that if the radical is a square root, it doesn’t have an index. If the radicals have the same index, or no index at all, multiply the numbers under the radical signs and put that number under it’s own radical symbol. Once you’ve multiplied the radicals, simplify your answer by attempting to break it down into a perfect square or cube. For tips on multiplying radicals that have coefficients or different indices, keep reading.
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