ACT Math : How to find a logarithm

Study concepts, example questions & explanations for ACT Math

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Example Questions

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Example Question #1 :Logarithms

Let log 5 = 0.69897 and log 2 = 0.30103. Solve log 50

Possible Answers:

1.68794

1.39794

1.69897

1.30103

1.36903

Correct answer:

1.69897

Explanation:

Using properties of logs:

log (xy) = logx+ logy

log (xn) =nlogx

log 10 = 1

So log 50 = log (10 * 5) = log 10 + log 5 = 1 + 0.69897 = 1.69897

Example Question #1 :Logarithms

y = 2x

If y = 3, approximately what is x?

Round to 4 decimal places.

Possible Answers:

1.5850

2.0000

1.3454

1.8580

0.6309

Correct answer:

1.5850

Explanation:

To solve, we use logarithms. We log both sides and get: log3 = log2x

which can be rewritten as log3 = xlog2

Then we solve for x: x = log 3/log 2 = 1.5850 . . .

Example Question #1 :Logarithms

Evaluate

log327

Possible Answers:

30

3

9

10

27

Correct answer:

3

Explanation:

You can change the form to

3x= 27

x= 3

Example Question #1 :How To Find A Logarithm

If\small \log_{x}49=2, what is\small x?

Possible Answers:

2401

24.5

0

7

10

Correct answer:

7

Explanation:

If\small \log_{x}y=z, then\small x^{z}=y

\small x^{2}=49

\small x=7

Example Question #1 :Logarithms

If log4x = 2, what is the square root of x?

Possible Answers:

12

4

3

2

16

Correct answer:

4

Explanation:

Given log4x= 2, we can determine that 4 to the second power isx; therefore the square root of x is 4.

Example Question #1 :Logarithms

Solve for x in the following equation:

log224 - log23 = logx27

Possible Answers:

9

3

1

2

2

Correct answer:

3

Explanation:

Since the two logarithmic expressions on the left side of the equation have the same base, you can use the quotient rule to re-express them as the following:

log224log23 = log2(24/3) = log28 = 3

Therefore we have the following equivalent expressions, from which it can be deduced thatx= 3.

logx27 = 3

x3= 27

Example Question #1 :Logarithms

What value ofsatisfies the equation?

Possible Answers:

Correct answer:

Explanation:

The answer is.

can by rewritten as.

In this form the question becomes a simple exponent problem. The answer isbecause.

Example Question #1 :Logarithms

If, what is?

Possible Answers:

Correct answer:

Explanation:

Use the following equation to easily manipulate all similar logs:

changes to.

Therefore,changes to.

2 raised to the power of 6 yields 64, somust equal 6. If finding the 6 was difficult from the formula, simply keep multiplying 2 by itself until you reach 64.

Example Question #1 :How To Find A Logarithm

Which of the following is a value ofxthat satisfies\log_{x}64=2?

Possible Answers:

32

16

2

8

4

Correct answer:

8

Explanation:

The general equation of a logarithm is\log_{x}a=b, andx^{b}=a

In this case,x^{2}=64, and thusx=8(or-8, but-8is not an answer choice)

Example Question #1 :How To Find A Logarithm

How can we simplify this expression below into a single logarithm?

Possible Answers:

Cannot be simplified into a single logarithm

Correct answer:

Explanation:

Using the property that, we can simplify the expression to.

Given thatand

We can further simplify this equation to

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