Exponents and Roots

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GRE Quantitative Reasoning › Exponents and Roots

Questions 1 - 10
1

If $x$ is a real number and $\sqrt{x}=\dfrac{1}{3}$, what is the value of $x^{3/2}$?

$\dfrac{1}{9}$

$\dfrac{1}{27}$

$\dfrac{1}{81}$

$\dfrac{3}{2}$

$\dfrac{1}{3}$

Explanation

This question tests composing exponents from root equations. The relevant rule is that ($\sqrt{x}$ = $x^{1/2}$ = rac{1}{3}), so (x = left( rac{1}{3} $ight)^2$ = rac{1}{9}). Then, $(x^{3/2}$ = $(x^{1/2}$$)^3$ = left( rac{1}{3} $ight)^3$ = rac{1}{27}). Alternatively, (left( rac{1}{9} $ight)^{3/2}$ = rac{1}{9} cdot $\sqrt{ rac{1}{9}$} = rac{1}{9} cdot rac{1}{3} = rac{1}{27}). The result is justified by consistent application of exponents. A distractor like choice A, ( rac{1}{9}), fails by stopping at (x) instead of applying the 3/2 exponent.

2

Which of the following is equivalent to $\left(\dfrac{1}{16}\right)^{3/4}$?

$\dfrac{1}{16}$

$\dfrac{1}{8}$

$\dfrac{1}{4}$

$8$

$\dfrac{1}{2}$

Explanation

This question tests evaluating fractional exponents on fractions. The relevant rule is that (left( rac{1}{16} $ight)^{3/4}$ = $(16^{-1}$$)^{3/4}$ = $16^{-3/4}$). Since (16 = $2^4$), $(16^{-3/4}$ = $(2^4$$)^{-3/4}$ = $2^{-3}$ = rac{1}{8}). Alternatively, (left( rac{1}{16} $ight)^{1/4}$ = rac{1}{2}), then raised to the third power is (left( rac{1}{2} $ight)^3$ = rac{1}{8}). The result is justified by consistent exponent simplification. A distractor like choice B, ( rac{1}{4}), fails by using an incorrect exponent like 1/2 instead of 3/4.

3

If $m$ and $n$ are positive integers such that $2^m=8^n$, what is the value of $\dfrac{m}{n}$?

$\dfrac{2}{3}$

$\dfrac{1}{3}$

$3$

$\dfrac{3}{2}$

$1$

Explanation

This question tests expressing numbers with the same base to find ratios of exponents. The relevant rule is to rewrite 8 as $(2^3$), so $(2^m$ = $(2^3$$)^n$ = $2^{3n}$). Equating exponents, (m = 3n), so ( rac{m}{n} = 3). This holds for positive integers (m) and (n). The result is justified as it satisfies the equation, like (m=3, n=1): $(2^3$ = $8^1$). A distractor like choice A, ( rac{1}{3}), fails by inverting the ratio, perhaps from miswriting (8 = $2^{1/3}$).

4

Which of the following is equal to $\dfrac{\sqrt{18}}{\sqrt{2}}$?

$\sqrt{9}$

$\sqrt{16}$

$6$

$3$

$9$

Explanation

This question tests simplifying ratios of square roots. The relevant rule is that ( rac{$\sqrt{18}$}{$\sqrt{2}$} = $\sqrt{ rac{18}{2}$} = $\sqrt{9}$). Simplifying further, ($\sqrt{9}$ = 3). Alternatively, factor as ( rac{3$\sqrt{2}$}{$\sqrt{2}$} = 3). The result is justified as it eliminates the roots correctly. A distractor like choice D, 9, fails by incorrectly squaring the entire expression or misapplying exponent rules.

5

Which of the following is equivalent to $\dfrac{\sqrt{x^6}}{x^2}$ for real $x\ne 0$?

$|x|$

$x^2$

$\dfrac{1}{|x|}$

$|x|^3$

$x$

Explanation

This question tests simplifying root expressions with absolute values for real numbers. The relevant rule is that $($\sqrt{x^6$}$ = $(x^6$$)^{1/2}$ = $|x^3$| = $|x|^3$). Dividing by $(x^2$) gives $(|x|^3$ / $x^2$ = $|x|^3$ / $|x|^2$ = |x|), since $(x^2$ = $|x|^2$). This holds for all real (x eq 0). The result is justified by checks like (x = -2): ($\sqrt{64}$/4 = 8/4 = 2 = |-2|). A distractor like choice A, (x), fails for negative (x), where it would give a negative instead of positive.

6

Which of the following is equivalent to $\dfrac{2^{5/2}}{2^{1/2}}$?

$\sqrt{2^4}$

$2^2$

$2^{5}$

$2^3$

$2^{2/2}$

Explanation

This question tests subtracting exponents with the same base. The relevant rule is that ( $rac{2^{5/2}$$}{2^{1/2}$} = $2^{5/2 - 1/2}$ = $2^{4/2}$ = $2^2$ = 4). This applies the quotient rule for exponents. The result equals 4, matching $(2^2$). The justification is the exponent arithmetic simplifying correctly. A distractor like choice D, $(2^{2/2}$ = $2^1$ = 2), fails by misapplying the fraction in the exponent.

7

Which of the following expressions is equivalent to $\left(16x^4\right)^{\frac{1}{2}}$ for real $x$?

$4x^2$

$4|x|$

$2x^2$

$|4x^2|$

$4x$

Explanation

This question tests the power rule for exponents and understanding of even roots. The rule states that $(a^m)^n = a^{mn}$. Applying this: $(16x^4)^{1/2} = 16^{1/2} \cdot(x^4)^{1/2} = 4 \cdot x^{4 \cdot 1/2} = 4x^2$. Note that $x^2$ is always non-negative for real $x$, so the expression $4x^2$ is well-defined. We can verify: when $x = 2$, $(16 \cdot 2^4)^{1/2} = (16 \cdot 16)^{1/2} = 256^{1/2} = 16$, and $4 \cdot 2^2 = 4 \cdot 4 = 16$ ✓. A common mistake would be to think $(x^4)^{1/2} = |x^2|$ or $2|x|$, not recognizing that $x^2$ is already non-negative.

8

Which of the following is true for all real numbers $t$ such that $t\neq 0$?

$\left(t\right)^{\frac{1}{2}}=\sqrt{|t|}$

$\left(t^2\right)^{\frac{1}{2}}=t$

$\left(t^2\right)^{\frac{3}{2}}=t^3$

$\left(t^3\right)^{\frac{1}{3}}=|t|$

$\left(t^4\right)^{\frac{1}{2}}=t^2$

Explanation

This question tests understanding of how fractional exponents behave with different bases. We need to check each option for all real $t \neq 0$. Option A: $(t^2)^{1/2} = |t|$, not $t$, since $\sqrt{t^2} = |t|$. Option B: $(t^3)^{1/3} = t$, not $|t|$, since cube roots preserve sign. Option C: $(t^4)^{1/2} = t^2$ is correct because $(t^4)^{1/2} = t^{4 \cdot 1/2} = t^2$, and this works for all real $t$. Option D: $(t^2)^{3/2} = |t|^3$, not $t^3$, when $t < 0$. Option E: $t^{1/2}$ is undefined for $t < 0$. Therefore, only option C is true for all real $t \neq 0$.

9

Which of the following expressions is equivalent to $\sqrt3{a^2},\sqrt3{a}$ for real $a$?

$a^2$

$a$

$\sqrt[3]{a^2}$

$\sqrt[3]{a}$

$\sqrt[3]{a^3}$

Explanation

This question tests the multiplication rule for radicals with the same index. The rule states that $\sqrt[n]{a} \cdot \sqrt[n]{b} = \sqrt[n]{ab}$ for real numbers where the radicals are defined. Applying this rule: $\sqrt[3]{a^2} \cdot \sqrt[3]{a} = \sqrt[3]{a^2 \cdot a} = \sqrt[3]{a^3}$. Since $\sqrt[3]{a^3} = a$ for all real $a$ (the cube root function is defined for all real numbers and preserves sign), the answer is $a$. A common mistake would be to add the exponents incorrectly, thinking $\sqrt[3]{a^2} \cdot \sqrt[3]{a} = \sqrt[3]{a^{2+1/3}}$, which misunderstands how radical multiplication works.

10

Which of the following is true for all real numbers $x$ such that the expressions are defined?

$\sqrt{x^2} = \ ?$

$\dfrac{1}{|x|}$

$\pm x$

$|x|$

$x^2$

$x$

Explanation

This question tests your understanding of the relationship between square roots and absolute values. The key principle is that $\sqrt{x^2} = |x|$ for all real numbers $x$. This is because squaring any real number (positive or negative) gives a non-negative result, and the square root function returns the non-negative value. For example, $\sqrt{(-3)^2} = \sqrt{9} = 3 = |-3|$. The expression equals the absolute value of $x$, not just $x$ itself, because when $x$ is negative, $\sqrt{x^2}$ gives the positive value. Choice A ($x$) would be incorrect for negative values of $x$, as it doesn't account for the sign change.

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