Logarithmic Functions

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ACT Math › Logarithmic Functions

Questions 1 - 10
1

A worksheet asks you to simplify $\log(2)+\log(50)$ (base 10). Which single logarithm is equivalent?

$\log!\left(\dfrac{2}{50}\right)$

$\log(100)$

$\log(52)$

$\log(2\cdot 50)\cdot\log(10)$

Explanation

This problem uses the logarithm product property. The product property states that log_a(x) + log_a(y) = log_a(xy). Applying this property to log(2) + log(50), we get log(2) + log(50) = log(2 × 50) = log(100). Choice A incorrectly adds the arguments instead of multiplying them.

2

A student wants a single logarithm equivalent to $\ln(12)-\ln(3)$. Which expression is equivalent?

$\ln(4)$

$\ln!\left(\dfrac{3}{12}\right)$

$\ln(9)$

$\ln(15)$

Explanation

This problem uses the logarithm quotient property. The quotient property states that $\ln(a) - \ln(b) = \ln(a/b)$. Applying this property to $\ln(12) - \ln(3)$, we get $\ln(12) - \ln(3) = \ln(12/3) = \ln(4)$. Choice A incorrectly keeps the arguments separate, while choice D incorrectly reverses the fraction.

3

What is $\log_{10}(1000)$?

1

2

3

10

Explanation

To evaluate this logarithm, we need to find what power 10 must be raised to get 1000. The logarithm property states that log_a(b) = c means $a^c$ = b. We can rewrite 1000 as $10^3$, so log₁₀(1000) = $log₁₀(10^3$). Using the power rule for logarithms, log_$a(x^n$) = n·log_a(x), we get 3·log₁₀(10) = 3·1 = 3.

4

Evaluate $\log_3(81)$.

2

3

4

5

Explanation

To evaluate this logarithm, we need to find what power 3 must be raised to get 81. The logarithm property states that $\log_a(b) = c$ means $a^c = b$. We can rewrite 81 as $3^4$ (since $3 \cdot 3 \cdot 3 \cdot 3 = 81$). Therefore, $\log_3(81) = \log_3(3^4)$. Using the power rule, this equals $4 \cdot \log_3(3) = 4 \cdot 1 = 4$.

5

A finance model uses natural logs. What is $\ln(e^5)$?

$5$

$e^5$

$\dfrac{1}{5}$

$5e$

Explanation

This problem uses the property that natural logarithm and exponential are inverse functions. The key property is that $ln(e^x$) = x for any real number x. Applying this property directly to $ln(e^5$), we get $ln(e^5$) = 5. Choice A incorrectly leaves the expression unsimplified, while choice B incorrectly multiplies by e.

6

A calculator app uses base-10 logs. If $\log(x)= -2$, what is the value of $x$?

$10^{-2}$

$2$

$-2$

$-10^{2}$

Explanation

Given log(x) = -2 (base 10 implied), we need to find x. Using the definition log₁₀(x) = -2 means 10^(-2) = x. Therefore, x = 10^(-2) = 1/10² = 1/100 = 0.01. Choice A gives just -2, which is the logarithm value, not x itself.

7

A student simplifies $\log_7(49)$. What is the value of $\log_7(49)$?

$7$

$49$

$2$

$\dfrac{1}{2}$

Explanation

This problem uses the fundamental relationship between logarithms and exponentials. The equation log_7(49) asks "to what power must we raise 7 to get 49?" Since $7^2$ = 49, we have log_7(49) = 2. Choice A incorrectly gives the reciprocal, while choice C gives the base instead of the exponent.

8

A student simplifies $\log_5(125)$. What is the value of $\log_5(125)$?

$3$

$5$

$125$

$2$

Explanation

This problem uses the fundamental relationship between logarithms and exponentials. The equation log_5(125) asks "to what power must we raise 5 to get 125?" Since $5^3$ = 125, we have log_5(125) = 3. Choice D incorrectly gives the argument of the logarithm rather than the exponent needed.

9

What is the value of $x$ if $\log(x) = 2$?

10

20

100

200

Explanation

To solve this equation, we need to convert from logarithmic to exponential form. The equation log(x) = 2 means "10 raised to what power equals x?" Since log without a specified base typically means log₁₀, we have 10² = x. Therefore, x = 100. Choice B would give 10¹ = 10, which doesn't satisfy the original equation.

10

Convert $\log_{3}(27) = y$ into exponential form.

$27^y = 3$

$3^3 = y$

$3^y = 27$

$y^3 = 27$

Explanation

To convert from logarithmic to exponential form, we use the fundamental property that log_a(b) = c is equivalent to $a^c$ = b. In the equation log₃(27) = y, the base is 3, the argument is 27, and the result is y. Converting to exponential form gives us $3^y$ = 27. Choice B incorrectly reverses the base and argument positions.

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