Tips for Multiplying Radicals

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A radical is basically a fractional exponent and is denoted by the radical sign (√). The expression x2 means to multiply x by itself (x • x), but when you see the expression √x, you're looking for a number that, when multiplied by itself, equals x. Similarly, 3√x means a number that, when multiplied by itself twice, equals x, and so on. Just as you can multiply numbers with the same exponent, you can do the same with radicals, as long as the superscripts in front of the radical signs are the same. For example, you can multiply (√x • √x) to get √(x2), which just equals x, and (3√x • 3√x) to get 3√(x2). However, the expression (√x • 3√x) can't be simplified any further.

Tip #1: Remember the "Product Raised to a Power Rule"

When multiplying exponents, the following is true: (a)x • (b)x = (a • b)x. The same rule applies when multiplying radicals. To see why, remember that you can express a radical as a fractional exponent. For example, √a = a1/2 or, in general, x√a = a1/x. When multiplying two numbers with fractional exponents, you can treat them the same as numbers with integral exponents, provided the exponents are the same. In general:

x√a • x√b = x√(a • b)

Example: Multiply √125 • √400

√25 • √400 = √(25 • 400) = √10,000

Tip #2: Simplify the Radicals before Multiplying Them

In the above example, you can quickly see that √125 = √52 = 5 and that √400 = √202 = 20 and that the expression simplifies to 100. That's the same answer you get when you look up the square root of 10,000.

In many cases, such as in the above example, it's easier to simplify numbers under the radical signs before you perform the multiplication. If the radical is a square root, you can remove numbers and variables that repeat in pairs from under the radical. If you're multiplying cube roots, you can remove numbers and variables that repeat in units of three. To remove a number from a fourth root sign, the number must repeat four times and so on.

Examples

1. Multiply √18 • √16

Factor the numbers under the radical signs and put any that occur twice outside the radical.

√18 = √(9 • 2) = √(3 • 3) • 2 = 3√2

√16 = √(4 • 4) = 4

√18 • √16 = 3√2 • 4 =

12√2

2. Multiply 3√(32x2 y4) • 3√(50x3y)

To simplify the cube roots, look for factors inside the radical signs that occur in units of three:

3√(32x2y4)= 3√(8 • 4)x2y4 = 3√[(2 • 2 • 2) • 4]x2 (y • y • y)y = 2y3√4x2y

3√(50 x3y) = 3√50 (x • x • x)y = x3√50y

The multiplication becomes

[2y(3√4x2y] • [x(3√50y)]

Multiplying like terms and applying the Product Raised to Power Rule, you get:

2xy • 3√(200x2y2)

References

About the Author

Chris Deziel holds a Bachelor's degree in physics and a Master's degree in Humanities, He has taught science, math and English at the university level, both in his native Canada and in Japan. He began writing online in 2010, offering information in scientific, cultural and practical topics. His writing covers science, math and home improvement and design, as well as religion and the oriental healing arts.

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