Solve: (√20 × √4)/√5 - Simplifying Square Root Fractions

Introduction:

We will address the following three laws of exponents:

a. Definition of root as an exponent:

$\sqrt[n]{a}=a^{\frac{1}{n}}$

b. The law of exponents for exponents applied to multiplication of terms in parentheses:

$(a\cdot b)^n=a^n\cdot b^n$

c. The law of exponents for exponents applied to division of terms in parentheses:

$(\frac{a}{b})^n=\frac{a^n}{b^n}$

Note:

(1). By combining the two laws of exponents mentioned in a (in the first and third steps ) and b (in the second step ), we can obtain a new rule:

$\sqrt[n]{a\cdot b}=\\ (a\cdot b)^{\frac{1}{n}}=\\ a^{\frac{1}{n}}\cdot b^{\frac{1}{n}}=\\ \sqrt[n]{a}\cdot \sqrt[n]{ b}\\ \downarrow\\ \boxed{\sqrt[n]{a\cdot b}=\sqrt[n]{a}\cdot \sqrt[n]{ b}}$

Specifically for the fourth root we obtain the following:

$\boxed{ \sqrt{a\cdot b}=\sqrt{a}\cdot \sqrt{ b}}$

(2). Note that by combining the two laws of exponents mentioned in a (in the first and third steps) and c (in the second step), we can obtain another new rule:

$\sqrt[n]{\frac{a}{b}}=\\ (\frac{a}{b})^{\frac{1}{n}}=\\ \frac{a^{\frac{1}{n}}}{ b^{\frac{1}{n}}}=\\ \frac{\sqrt[n]{a}}{ \sqrt[n]{ b}}\\ \downarrow\\ \boxed{ \sqrt[n]{\frac{a}{b}}=\frac{\sqrt[n]{a}}{ \sqrt[n]{ b}}}$

Specifically for the fourth root we obtain the following:

$\boxed{ \sqrt{\frac{a}{ b}}=\frac{\sqrt{a}}{\sqrt{ b}} }$

Therefore, in solving the problem, meaning - in simplifying the given expression, we will use the two new rules that we studied in the introduction:

(1).

$\boxed{ \sqrt{a\cdot b}=\sqrt{a}\cdot \sqrt{ b}}$

(2).

$\boxed{ \sqrt{\frac{a}{ b}}=\frac{\sqrt{a}}{\sqrt{ b}} }$

We'll start by simplifying the expression in the numerator using the rule that we studied in the introduction (1) (however this time in the opposite direction, meaning we'll insert the multiplication of roots as a multiplication of terms under the same root) Then we'll proceed to perform the multiplication under the root in the numerator:

$\frac{\sqrt{20}\cdot\sqrt{4}}{\sqrt{5}}= \\ \frac{\sqrt{20\cdot4}}{\sqrt{5}}= \\ \frac{\sqrt{80}}{\sqrt{5}}= \\$We'll continue to simplify the fraction, using the rule we received in the introduction (2) (however in the opposite direction, meaning we'll insert the division of roots as a division of terms under the same root) Then we'll proceed to reduce the fraction under the root:

$\frac{\sqrt{80}}{\sqrt{5}}= \\ \\ \sqrt{\frac{80}{5}}=\\ \sqrt{16}=\\ \boxed{4}$

In the final stage, after reducing the fraction under the root, we used the known fourth root of the number 16.

Let's summarize the simplification process of the expression in the problem:

$\frac{\sqrt{20}\cdot\sqrt{4}}{\sqrt{5}}= \\ \frac{\sqrt{80}}{\sqrt{5}}= \\ \sqrt{16}=\\ \boxed{4}$

Therefore, the correct answer is answer B.