SAT II Math II : Exponents and Logarithms

Study concepts, example questions & explanations for SAT II Math II

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Example Questions

Example Question #21 : Mathematical Relationships

Solve \displaystyle ln x - ln 4 = 5

Possible Answers:

\displaystyle 4+e^5

\displaystyle 1024e^5

\displaystyle 1024+e^5

\displaystyle 4e^5

\displaystyle 1024e

Correct answer:

\displaystyle 4e^5

Explanation:

First, subtract the natural log terms:

\displaystyle ln(\frac{x}{4})=5

Now rewrite the equation in exponential form:

\displaystyle e^5=\frac{x}{4}

Finally, isolate the variable:

\displaystyle 4e^5=x

Example Question #21 : Exponents And Logarithms

Solve \displaystyle log_{18} (2 - x) = log_{18} (-2x - 2)

Possible Answers:

\displaystyle 3

\displaystyle -2

\displaystyle 2

\displaystyle -4

\displaystyle 0

Correct answer:

\displaystyle -4

Explanation:

We can start by canceling the logs, because they both have the same base:

\displaystyle 2-x=-2x-2

Now we can collect constants on one side of the equation, and variables on the other:

\displaystyle x=-4

Example Question #31 : Mathematical Relationships

Solve \displaystyle -log_7(x - 7) + 1 = -1.

Possible Answers:

\displaystyle 56

\displaystyle 28

\displaystyle 21

\displaystyle 49

\displaystyle -7

Correct answer:

\displaystyle 56

Explanation:

We can start by gathering all the constants to one side of the equation:

\displaystyle -log_7(x - 7) = -2

Next, we can multiply by \displaystyle -1 to change the signs:

\displaystyle log_7(x - 7) = 2

Now we can rewrite the equation in exponential form:

\displaystyle 7^2=x-7

And finally, we can solve algebraically:

\displaystyle 49=x-7

\displaystyle 56=x

Example Question #22 : Exponents And Logarithms

Solve for \displaystyle x:

\displaystyle 81^{x-3}=27^{2x}

Possible Answers:

\displaystyle x=-5

\displaystyle x=-4

\displaystyle x=2

\displaystyle x=-6

Correct answer:

\displaystyle x=-6

Explanation:

In order to solve this problem, rewrite both sides of the equation in terms of raising \displaystyle 3 to an exponent.

Since, \displaystyle 81=3^4, we can write the following:

\displaystyle 81^{x-3}=(3^4)^{x-3}=3^{4x-12}

Since \displaystyle 27=3^3, we can write the following:

\displaystyle 27^{2x}=(3^3)^{2x}=3^{6x}

Now, we can solve for \displaystyle x with the following equation:

\displaystyle 6x=4x-12

\displaystyle 2x=-12

\displaystyle x=-6

 

Example Question #22 : Exponents And Logarithms

Solve 

\displaystyle -9e^{x + 1} = 47

Possible Answers:

\displaystyle ln(\frac{3}{4})

\displaystyle 17

\displaystyle \frac{3}{e}

\displaystyle \frac{e}{4}

No solutions

Correct answer:

No solutions

Explanation:

The first thing we need to do is find a common base. However, because one of the bases has an \displaystyle e in it (an irrational number), and the other does not, it's going to be impossible to find a common base. Therefore, the question has no solution.

Example Question #23 : Exponents And Logarithms

Solve \displaystyle log_{15} (48 + x) = log_{15} (x^2 + 3x)

Possible Answers:

No solutions

\displaystyle -8, -6

\displaystyle -6, 8

\displaystyle -8, 6

\displaystyle 6, 8

Correct answer:

\displaystyle -8, 6

Explanation:

First, we can simplify by canceling the logs, because their bases are the same:

\displaystyle 48+x=x^2+3x

Now we collect all the terms to one side of the equation:

\displaystyle x^2+2x-48=0

Factoring the expression gives:

\displaystyle (x-6)(x+8)=0

So our answers are:

\displaystyle x=-8, 6

Example Question #23 : Exponents And Logarithms

Solve \displaystyle 25^x = 625^x.

Possible Answers:

\displaystyle 2

\displaystyle e

\displaystyle 0

\displaystyle x^2

No solutions

Correct answer:

\displaystyle 0

Explanation:

Here, we can see that changing base isn't going to help.  However, if we remember that and number raised to the \displaystyle 0th power equals \displaystyle 1, our solution becomes very easy.

\displaystyle 25^{(0)}=625^{(0)}

\displaystyle 1=1

 

Example Question #24 : Exponents And Logarithms

To the nearest hundredth, solve for \displaystyle x\displaystyle 7^{2x} = 5 ^{x}.

Possible Answers:

\displaystyle 2.28

\displaystyle 0.99

None of these

\displaystyle 0.55

\displaystyle 1.27

Correct answer:

None of these

Explanation:

Take the natural logarithm of both sides:

\displaystyle 7^{2x} = 5 ^{x}

\displaystyle \ln 7^{2x} = \ln 5 ^{x}

By the Logarithm of a Power Rule the above becomes

\displaystyle 2x \ln 7 = x \ln 5

Solve for \displaystyle x:

\displaystyle 2x \ln 7 - x \ln 5 = x \ln 5 - x \ln 5

\displaystyle (2 \ln 7 - \ln 5) x = 0

\displaystyle \frac{(2 \ln 7 - \ln 5) x }{2 \ln 7 - \ln 5}= \frac{0 }{2 \ln 7 - \ln 5}

\displaystyle x = 0.

This is not among the choices given.

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