Using Logarithm Properties

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Algebra 2 › Using Logarithm Properties

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1

Use the logarithm properties

<u>\log_b(xy)=\log_b(x)+\log_b(y),\quad \log_b\left(\frac{x}{y}\right)=\log_b(x)-\log_b(y),\quad \log_b(x^p)=p\log_b(x)</u>

to evaluate without a calculator: $$\log_3\left(\frac{81}{9}\right).$$

$4$

$2$

$\tfrac{1}{2}$

$0$

Explanation

This question tests your understanding of the quotient property of logarithms and your ability to evaluate logs by recognizing powers of the base. The quotient property states that log_b(x/y) = log_b(x) - log_b(y), which means you can split a fraction inside a log into a difference, or evaluate it directly if you recognize the result as a power of the base. To evaluate log_3(81/9): First, calculate the fraction: 81/9 = 9. Then evaluate log_3(9): Since 9 = 3², we have log_3(9) = log_3(3²) = 2. Alternatively, using the quotient property: log_3(81/9) = log_3(81) - log_3(9) = log_3(3⁴) - log_3(3²) = 4 - 2 = 2. Choice C correctly gives 2 as the answer, whether you simplify first or use the quotient property. Choice A (1/2) would mean 3^(1/2) = 81/9, but 3^(1/2) = √3 ≈ 1.73, not 9. The key insight is recognizing powers of 3: 3¹ = 3, 3² = 9, 3³ = 27, 3⁴ = 81. Evaluation strategy for quotients: (1) You can simplify the fraction first if possible (81/9 = 9), then evaluate the simpler log, (2) Or apply quotient property and evaluate each log separately, (3) Both methods give the same answer—choose based on which recognition is easier. Mental math tip: memorize small powers of common bases (2, 3, 5, 10) to quickly evaluate many logarithms without a calculator!

2

Use log properties to expand the expression: $$\log_2\left(\frac{(xy^2)^3}{\sqrt{z}}\right).$$

$3\log_2(x)+2\log_2(y)-\frac{1}{2}\log_2(z)$

$3\log_2(x)+6\log_2(y)-\frac{1}{2}\log_2(z)$

$\log_2(x)+\log_2(y)-\log_2(z)$

$\log_2(x+y)-\log_2(z)$

Explanation

This question tests expanding complex logs with properties. Properties simplify nested expressions! To expand $ \log_2 \left( \frac{(x y^2)^3}{\sqrt{z}} \right) $: (1) Quotient: $ \log_2( (x y^2)^3 ) - \log_2( z^{1/2} ) $. (2) Power: $ 3 \log_2( x y^2 ) - \frac{1}{2} \log_2(z) $. (3) Product and power inside: $ 3 (\log_2(x) + 2 \log_2(y)) - \frac{1}{2} \log_2(z) = 3 \log_2(x) + 6 \log_2(y) - \frac{1}{2} \log_2(z) $. Choice A correctly expands. Choice B uses 2 instead of 6 for y—remember to distribute the 3 to y^2's log! Strategy: (1) Outermost (quotient). (2) Outer power. (3) Inner product/power. Work inside out—you're excelling!

3

A chemistry student uses the model $\text{pH}=-\log(H^+)$ (base 10). If $H^+=10^{-7}$, what is the pH?

(Use the power property $\log(10^p)=p$.)​

$7$

$0$

$-7$

$\tfrac{1}{7}$

Explanation

This question tests your understanding of the three fundamental logarithm properties—product, quotient, and power rules—that let you expand complex logarithmic expressions into sums and differences or condense multiple logs into a single logarithm. The power property log_$b(x^p$) = p·log_b(x) is particularly useful here, and when the base of the logarithm matches the base inside, we get the special case log_$b(b^p$) = p directly! To find pH when [H⁺] = 10⁻⁷: (1) Substitute into the formula: pH = -log(10⁻⁷), (2) Apply the power property: -log(10⁻⁷) = -(-7)·log(10), (3) Since log(10) = 1 (base 10 logarithm of 10 equals 1), we get: -(-7)·1 = 7, (4) Therefore pH = 7. Alternatively, use $log(10^p$) = p directly: pH = -log(10⁻⁷) = -(-7) = 7. Choice B correctly gives pH = 7. Choice A (-7) forgets to apply the negative sign in the pH formula—the formula is pH = -log([H⁺]), not pH = log([H⁺]), so we need to negate the logarithm result! The negative sign in the formula is crucial because [H⁺] values are typically very small (like 10⁻⁷), giving negative logarithms, but pH scale uses positive numbers. Real-world logarithm application strategy: (1) Identify the logarithm formula and what each variable represents, (2) Substitute given values carefully, including signs, (3) Use properties to simplify, especially when you see powers of 10 with base-10 logarithms, (4) Remember $log₁₀(10^n$) = n makes calculations straightforward. The pH scale's use of negative logarithm transforms tiny decimal concentrations (0.0000001) into manageable numbers (7) while reversing the scale so lower [H⁺] means higher pH!

4

Use the logarithm properties

<u>\log_b(xy)=\log_b(x)+\log_b(y),\quad \log_b\left($\frac{x}{y}$\right)=\log_b(x)-\log_b(y),\quad \log_$b(x^p$)=p\log_b(x)</u>

to expand the expression as a sum and/or difference of logarithms:

<u>\log_$3\left($\frac{x^2y$$}{z^3$}$\right).</u>

$\log_3(x^2)+\log_3(y)+\log_3(z^3)$

$\log_3(x^2+y)-\log_3(z^3)$

$\log_3(x)+\log_3(y)-\log_3(z)$

$2\log_3(x)+\log_3(y)-3\log_3(z)$

Explanation

This question tests your understanding of the three fundamental logarithm properties—product, quotient, and power rules—that let you expand complex logarithmic expressions into sums and differences. The three logarithm properties come from exponent rules and work because logarithms are exponents: (1) Product property: log_b(xy) = log_b(x) + log_b(y) because multiplying numbers means adding their exponents, (2) Quotient property: log_b(x/y) = log_b(x) - log_b(y) because dividing means subtracting exponents, (3) Power property: log_$b(x^p$) = p log_b(x) because raising to power means multiplying exponents. To expand log_$3((x^2$ $y)/z^3$), first apply the quotient property: log_$3(x^2$ y) - log_$3(z^3$), then apply the product property to the numerator: log_$3(x^2$) + log_3(y) - log_$3(z^3$), and finally the power property: 2 log_3(x) + log_3(y) - 3 log_3(z). Choice B correctly applies these properties in sequence to fully expand the expression into a sum and difference of simpler logs with coefficients. Choice D incorrectly combines $x^2$ + y inside a log instead of using the product property for $x^2$ y, remember there's no property for log of a sum—it's a common mix-up with the product rule! When expanding logarithms, start with the outermost operation like quotients or overall powers, then break down products and individual powers step by step. This systematic approach ensures you don't miss any terms—keep practicing, you've got this!

5

Condense into a single logarithm (natural log):

<u>\t$\frac{3}{2}$\ln(x)+\ln(y)-\t$\frac{1}{2}$\ln(z).</u>

$\ln\left(\frac{(3/2)xy}{(1/2)z}\right)$

$\ln\left(\frac{xy}{z}\right)^{1/2}$

$\ln\left(\frac{x^3y}{z}\right)$

$\ln\left(\frac{x^{3/2}y}{z^{1/2}}\right)$

Explanation

This question tests your understanding of the three fundamental logarithm properties—product, quotient, and power rules—that let you condense sums and differences into a single logarithm. The three logarithm properties come from exponent rules and work because logarithms are exponents: (1) Product property: ln(x) + ln(y) = ln(xy), (2) Quotient property: ln(x) - ln(y) = ln(x/y), (3) Power property: p ln(x) = $ln(x^p$). To condense (3/2) ln(x) + ln(y) - (1/2) ln(z), first power: $ln(x^{3/2}$) + ln(y) - $ln(z^{1/2}$), then product: $ln(x^{3/2}$ y) - $ln(z^{1/2}$), then quotient: ln( $(x^{3/2}$ y) / $z^{1/2}$ ). Choice A correctly applies the reverse properties to form the condensed expression. Choice B treats coefficients as multipliers inside instead of exponents—power rule pulls them up! Handle fractional coefficients with power first, then combine step by step. You're getting the hang of this—superb!

6

Use the logarithm properties $\log_b(xy)=\log_b(x)+\log_b(y)$, $\log_b\left(\frac{x}{y}\right)=\log_b(x)-\log_b(y)$, and $\log_b(x^p)=p\log_b(x)$ to expand the expression: $$\log_5\left(\frac{x^3\sqrt{y}}{z^2}\right).$$ Write your answer as a sum/difference of logarithms.

$\log_5\left(\frac{x+y}{z}\right)$

$\log_5(x^3)+\log_5(\sqrt{y})-\log_5(z)$

$3\log_5(x)+\frac{1}{2}\log_5(y)-2\log_5(z)$

$\log_5(x^3)+\log_5(\sqrt{y})+\log_5(z^2)$

Explanation

This question tests your understanding of the three fundamental logarithm properties—product, quotient, and power rules—that let you expand complex logarithmic expressions into sums and differences. The three logarithm properties come from exponent rules and work because logarithms ARE exponents: (1) Product property: $\log_b(xy) = \log_b(x) + \log_b(y)$ because multiplying numbers means adding their exponents, (2) Quotient property: $\log_b(x/y) = \log_b(x) - \log_b(y)$ because dividing means subtracting exponents, (3) Power property: $\log_b(x^p) = p \log_b(x)$ because raising to power means multiplying exponents. These properties transform multiplication into addition, division into subtraction, and exponentiation into multiplication—powerful simplification tools! To expand $\log_5((x^3 \sqrt{y})/z^2)$ using properties: (1) Apply quotient property first: $\log_5(x^3 \sqrt{y}) - \log_5(z^2)$ because the overall structure is a fraction. (2) Apply product property to first log: $\log_5(x^3) + \log_5(\sqrt{y})$ because $x^3 \sqrt{y}$ is a product. (3) Apply power property to each: $3\log_5(x) + \frac{1}{2}\log_5(y) - 2\log_5(z)$ because $x^3$ has exponent 3, $\sqrt{y}$ is $y^{1/2}$, and $z^2$ has exponent 2. Final expanded form: $3\log_5(x) + \frac{1}{2}\log_5(y) - 2\log_5(z)$. Choice A correctly applies the logarithm properties in the right order to expand into sum and difference of simpler logs. Choice B incorrectly keeps the logs condensed without fully expanding using the power property, leaving exponents inside instead of bringing them out as coefficients—remember to apply power rule to complete the expansion! Expanding logarithm strategy: (1) Identify overall structure: is it product, quotient, or power? Start with outermost operation. (2) Apply matching property: multiplication becomes +, division becomes -, exponent becomes coefficient. (3) Work from outside in: handle fraction first (quotient property), then products within (product property), then exponents (power property). (4) Continue until each log has single variable with coefficient. You're doing great—keep practicing this step-by-step approach to master log expansions!

7

Use the logarithm properties

<u>\log_b(xy)=\log_b(x)+\log_b(y),\quad \log_b\left(\frac{x}{y}\right)=\log_b(x)-\log_b(y),\quad \log_b(x^p)=p\log_b(x)</u>

to expand the expression $\log\left(\frac{(mn)^4}{p}\right)$ (common log base 10) into a sum/difference of logs with no exponents inside the logs.

$4\log(m)+4\log(n)-\log(p)$

$4\log(mn)-\log(p)$

$\log(m+n)^4-\log(p)$

$\log(m)+\log(n)-\log(p^4)$

Explanation

This question tests your understanding of the three fundamental logarithm properties—product, quotient, and power rules—that let you expand complex logarithmic expressions into sums and differences with no exponents inside the logs. The three properties are: (1) Product property: log(xy) = log(x) + log(y), (2) Quotient property: log(x/y) = log(x) - log(y), (3) Power property: $log(x^p$) = p·log(x)—this one is crucial for removing exponents from inside logs. To expand log((mn)⁴/p): First apply the quotient property to the overall fraction: log((mn)⁴) - log(p). Next apply the power property to the first term: 4log(mn) - log(p), bringing the exponent 4 down as a coefficient. Then apply the product property inside: 4[log(m) + log(n)] - log(p). Finally distribute the 4: 4log(m) + 4log(n) - log(p). Choice B correctly shows the fully expanded form with no exponents inside any logarithm and the coefficient 4 distributed to both m and n. Choice A incorrectly leaves mn together inside the log instead of separating them—when you have 4log(mn), you must apply the product property to get 4log(m) + 4log(n), not just 4log(mn). The key insight is that (mn)⁴ = m⁴n⁴, so log((mn)⁴) = log(m⁴n⁴) = log(m⁴) + log(n⁴) = 4log(m) + 4log(n). Expansion strategy for removing all exponents: (1) Handle the overall structure first (quotient here), (2) Bring down any exponents as coefficients using power property, (3) Expand any products or quotients inside, (4) Distribute coefficients if needed. The goal is to have each log contain only a single variable with no exponent, with any powers appearing as coefficients in front!

8

Use the logarithm properties

<u>\log_b(xy)=\log_b(x)+\log_b(y),\quad \log_b\left(\frac{x}{y}\right)=\log_b(x)-\log_b(y),\quad \log_b(x^p)=p\log_b(x)</u>

to write as a single logarithm: $$\log_2(5)+\log_2(3)-\log_2(4).$$

$\log_2\left(\dfrac{15}{8}\right)$

$\log_2\left(\dfrac{5}{12}\right)$

$\log_2\left(\dfrac{15}{4}\right)$

$\log_2(5+3-4)$

Explanation

This question tests your understanding of the three fundamental logarithm properties—product, quotient, and power rules—that let you condense sums and differences of logarithms into a single logarithm. The properties for condensing are: (1) Product property backwards: log_b(x) + log_b(y) = log_b(xy), so addition becomes multiplication, (2) Quotient property backwards: log_b(x) - log_b(y) = log_b(x/y), so subtraction becomes division. To condense log_2(5) + log_2(3) - log_2(4): First combine the addition: log_2(5) + log_2(3) = log_2(5·3) = log_2(15). Then handle the subtraction: log_2(15) - log_2(4) = log_2(15/4). Choice A correctly shows the condensed form as log_2(15/4), a single logarithm of the fraction 15/4. Choice C incorrectly tries to combine using 5 + 3 - 4 inside the log, but there is NO logarithm property that allows log(a) + log(b) - log(c) = log(a + b - c). You must use multiplication for addition of logs and division for subtraction of logs! Condensing strategy: (1) Work left to right combining two logs at a time, (2) Addition of logs → multiply the arguments, (3) Subtraction of logs → divide the arguments, (4) Never add or subtract the arguments directly. The pattern to remember: plus outside means times inside, minus outside means divide inside!

9

Use the product, quotient, and power properties of logarithms to write the following as a single logarithm (base 10): $$2\log(x) + \frac{1}{2}\log(y) - 3\log(z).$$

$\log\left(x^2 y^{1/2} z^3\right)$

$\log\left(\frac{x^2 + \sqrt{y}}{z^3}\right)$

$\log\left(\frac{x^2 y^{1/2}}{z^3}\right)$

$\log\left(\frac{x^2 y}{z^3}\right)$

Explanation

This question tests your understanding of the three fundamental logarithm properties—product, quotient, and power rules—that let you condense multiple logarithmic expressions into a single logarithm. The three logarithm properties come from exponent rules and work because logarithms ARE exponents: (1) Product property: $ \log_b(xy) = \log_b(x) + \log_b(y) $ because multiplying numbers means adding their exponents, (2) Quotient property: $ \log_b(x/y) = \log_b(x) - \log_b(y) $ because dividing means subtracting exponents, (3) Power property: $ \log_b(x^p) = p \log_b(x) $ because raising to power means multiplying exponents. These properties transform addition into multiplication, subtraction into division, and coefficients into exponents—powerful simplification tools! For condensing $ 2\log(x) + \frac{1}{2}\log(y) - 3\log(z) $, reverse the properties: (1) Apply power property backwards: $ \log(x^2) + \log(y^{1/2}) - \log(z^3) $ by bringing coefficients up as exponents. (2) Apply product property backwards to the sum: $ \log(x^2 y^{1/2}) - \log(z^3) $ because sum of logs becomes log of product. (3) Apply quotient property backwards: $ \log\left( \frac{x^2 y^{1/2}}{z^3} \right) $ because difference becomes quotient. Final condensed form: $ \log\left( \frac{x^2 \sqrt{y}}{z^3} \right) $. Choice A correctly applies the logarithm properties in the right order to condense into a single logarithm. Choice B incorrectly ignores the 1/2 exponent on y, treating it as $ \log(y) $ instead of $ \log(\sqrt{y}) $—always remember to apply the power rule first to coefficients! Condensing strategy: (1) Use power property forward: coefficient in front becomes exponent (like $ 2\log(x) $ to $ \log(x^2) $). (2) Identify additions: $ \log(a) + \log(b) $ condenses to $ \log(a b) $. (3) Identify subtractions: $ \log(a) - \log(b) $ condenses to $ \log(a/b) $. (4) Combine all step-by-step into one log. Great job—keep building your skills with this reverse process!

10

Using the properties

$$\log_b(xy)=\log_b(x)+\log_b(y),\quad \log_b\left(\frac{x}{y}\right)=\log_b(x)-\log_b(y),\quad \log_b(x^p)=p\log_b(x),$$

write the expression as a single logarithm (condense):

$$2\log_5(a)-\tfrac{1}{2}\log_5(b)+\log_5(c).$$

$\log_5\left(\frac{a^2\sqrt{b}}{c}\right)$

$\log_5(a^2)+\log_5(c)-\log_5(\sqrt{b})$

$\log_5\left(\frac{a^2c}{\sqrt{b}}\right)$

$\log_5\left(\frac{a^2c}{b^2}\right)$

Explanation

This question tests your understanding of the three fundamental logarithm properties—product, quotient, and power rules—that let you expand complex logarithmic expressions into sums and differences or condense multiple logs into a single logarithm. The three logarithm properties come from exponent rules and work because logarithms ARE exponents: (1) Product property: $log_b(xy) = log_b(x) + log_b(y)$ because multiplying numbers means adding their exponents, (2) Quotient property: $log_b(x/y) = log_b(x) - log_b(y)$ because dividing means subtracting exponents, (3) Power property: $log_b(x^p) = p\cdot log_b(x)$ because raising to power means multiplying exponents. To condense $2log_5(a) - (1/2)log_5(b) + log_5(c)$, we work backwards: (1) Apply power property backwards: $2log_5(a)$ becomes $log_5(a^2)$ and $(1/2)log_5(b)$ becomes $log_5(b^(1/2)) = log_5(\sqrt{b})$, (2) Rewrite: $log_5(a^2) - log_5(\sqrt{b}) + log_5(c)$, (3) Apply quotient property backwards to first two: $log_5(a^2/\sqrt{b}) + log_5(c)$, (4) Apply product property backwards: $log_5((a^2/\sqrt{b})\cdot c) = log_5(a^2c/\sqrt{b})$. Choice A correctly condenses the expression into a single logarithm by applying properties in reverse order. Choice B incorrectly has $\sqrt{b}$ in the numerator instead of denominator—remember that subtraction of logs means division, so $-log_5(\sqrt{b})$ puts $\sqrt{b}$ in the denominator! Condensing strategy (reverse process): (1) Use power property backwards: coefficient in front becomes exponent ($2log(x)$ becomes $log(x^2)$), (2) Identify subtractions: $log(a) - log(b)$ condenses to $log(a/b)$ using quotient property backwards, (3) Identify additions: $log(a) + log(b)$ condenses to $log(ab)$ using product property backwards, (4) Combine all: the result should be single log of one big expression. Work step-by-step combining two logs at a time, and be careful with signs—minus means divide!

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