GRE Subject Test: Math : Algebra

Study concepts, example questions & explanations for GRE Subject Test: Math

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Example Questions

Example Question #131 : Algebra

\(\displaystyle Solve\ for\ x: log_{3}(x)=2\)

Possible Answers:

\(\displaystyle 3\)

\(\displaystyle \frac{1}{3}\)

\(\displaystyle 6\)

\(\displaystyle 9\)

Correct answer:

\(\displaystyle 9\)

Explanation:

\(\displaystyle We\ can\ use\ the\ general\ rule:\ log_{b}(a)=x\\ when\ rewritten\ in\ exponential\ form\ we\ get: b^{x}=a\)

\(\displaystyle In\ this\ case\ we\ have\ log_{3}(x)=2\)

\(\displaystyle This\ can\ be\ rewritten\ in\ exponential\ form\ as: 3^2=x\)

\(\displaystyle We\ know\ that\ 3^2=9;\ so\ x=9\)

 

Example Question #4 : Logarithmic Properties

\(\displaystyle Rewrite\ as\ one\ log:\ log(a)-(\left log(b)+log(c) \right)\)

Possible Answers:

\(\displaystyle log\left ( \frac{a}{bc} \right )\)

\(\displaystyle log(a-b-c)\)

\(\displaystyle log(a-b+c)\)

\(\displaystyle log\left ( \frac{ab}{c} \right )\)

Correct answer:

\(\displaystyle log\left ( \frac{a}{bc} \right )\)

Explanation:

\(\displaystyle When\ condensing\ logs\ we\ must\ remember\ the\ log\ properties:\)

\(\displaystyle log(x)+log(y)=log(xy): When\ adding;\ we\ multiply.\)

\(\displaystyle log(x)-log(y)=log\left (\frac{x}{y} \right ): When\ subtracting;\ we\ divide.\)

\(\displaystyle GIVEN: Rewrite\ as\ one\ log:\ log(a)-(\left log(b)+log(c) \right)\\\)

\(\displaystyle This\ is\ rewritten\ as:\ log \left (\frac{a}{bc} \right )\\ because\ the\ negative\ will\ be\ distributed\ to\ log(b)\ and\ log(c)\)

Example Question #2 : Logarithmic Properties

Rewrite the following expression as a single logarithm

\(\displaystyle 5log_35x+log_3\frac{1}{4x^3}-log_3x^2\)

Possible Answers:

\(\displaystyle log_3\frac{3125x^3}{4}\)

\(\displaystyle log_3\frac{3125x^3}{9}\)

\(\displaystyle 6.06\)

\(\displaystyle 3.076\)

\(\displaystyle log_33125x\)

Correct answer:

\(\displaystyle 6.06\)

Explanation:

Recall a few properties of logarithms:

1.When adding logarithms of like base, we multiply the inside.

2.When subtracting logarithms of like base, we divide the inside.

3. When multiplying a logarithm by a number, we can raise the inside to that power.

So we begin with this:

\(\displaystyle 5log_35x+log_3\frac{1}{4x^3}-log_3x^2\)

I would start with 3 to simplify the first log.

\(\displaystyle log_3(5x)^5+log_3\frac{1}{4x^3}-log_3x^2\)

\(\displaystyle log_33125x^5+log_3\frac{1}{4x^3}-log_3x^2\)

Next, use rule 1 on the first two logs.

\(\displaystyle log_3\left(3125x^5\cdot\frac{1}{4x^3}\right)-log_3x^2\)

\(\displaystyle log_3\left(\frac{3125x^2}{4}\right)-log_3x^2\)

Then, use rule 2 to combine these two.

\(\displaystyle log_3\left(\frac{3125x^2}{4x^2}\right)=log_3\left(\frac{3125}{4}\right)\approx 6.06\)

So our answer is 6.06.

Example Question #1 : Change Of Base

Evaluate:  

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

Possible Answers:

\(\displaystyle -\frac{1}{2}\)

\(\displaystyle \frac{4}{13}\)

\(\displaystyle -1\)

\(\displaystyle \frac{1}{2}\)

\(\displaystyle \frac{2}{3}\)

Correct answer:

\(\displaystyle -1\)

Explanation:

In order to evaluate the unknown variable, it is necessary to change the base. Looking at the right side of the equation, 27 is equivalent to three cubed.

\(\displaystyle 3^3=27\)

Therefore, converting the right side of the equation to a base of 3 will allow setting both the left and right side of the exponential terms equal to each other.

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

\(\displaystyle 3^{3x+3}=3^{3(2x+2)}\)

Log both sides to drop the exponents by log properties, and divide the log based 3 on both sides to cancel this term.  

Solve for x.

\(\displaystyle ln\: 3^(^3^x^+^3^)=ln\:3^3^(^2^x^+^2^)\)

\(\displaystyle 3x+3=3(2x+2)\)

\(\displaystyle 3x+3=6x+6\)

\(\displaystyle -3=3x\)

\(\displaystyle x=-1\)

Example Question #1 : Change Of Base

Evaluate the following logarithm:

\(\displaystyle log_{377}(563)\)

Possible Answers:

\(\displaystyle 1.238\)

\(\displaystyle 0.982\)

\(\displaystyle 0.937\)

\(\displaystyle 1.068\)

\(\displaystyle 4.652\)

Correct answer:

\(\displaystyle 1.068\)

Explanation:

The simplest way to evaluate a logarithm that doesn't have base 10 is with change of base formula:

\(\displaystyle log_ba=\frac{log_ca}{log_cb}\)

So we have

\(\displaystyle log_{377}(563)\)

\(\displaystyle log_{377}563=\frac{log_{10}563}{log_{10}377}\approx1.068\)

Example Question #3 : Change Of Base

\(\displaystyle Solve\ for\ x: 3^x=23\)

Possible Answers:

\(\displaystyle 2.85405\)

\(\displaystyle 2.73058\)

\(\displaystyle 1.89205\)

\(\displaystyle 1.683099\)

Correct answer:

\(\displaystyle 2.85405\)

Explanation:

In order to solve a logarithm, we must first rewrite it in log form: 

\(\displaystyle 3^x=23\ written\ in\ log\ form\ is: log_{3}(23)=x\)

To solve for x, we must use the Change of Base: 

\(\displaystyle log_{3}(23)=\frac{log(23)}{log(3)}\)

\(\displaystyle \frac{log(23)}{log(3)}=2.85405\)

This means that: 

\(\displaystyle 3^{2.85405}=23\)

Example Question #1 : Solving Exponential Equations

Find one possible value of \(\displaystyle t\), given the following equation:

\(\displaystyle 343=7^{15t-12}\)

Possible Answers:

Cannot be determined from the information given.

\(\displaystyle t=0\)

\(\displaystyle t=-3\)

\(\displaystyle t=5\)

\(\displaystyle t=1\)

Correct answer:

\(\displaystyle t=1\)

Explanation:

We begin with the following:

\(\displaystyle 343=7^{15t-12}\)

This can be rewritten as

\(\displaystyle 7^3=7^{15t-12}\)

Recall that if you have two exponents with equal bases, you can simply set the exponents equal to eachother. Do so to get the following:

\(\displaystyle 3=15t-12\)

Solve this to get t.

\(\displaystyle 15=15t\)

\(\displaystyle t=1\)

Example Question #132 : Algebra

Solve for \(\displaystyle x\).

\(\displaystyle 4^{2x}=16^{2x-6}\)

Possible Answers:

\(\displaystyle x=log \ 2\)

\(\displaystyle x=4\)

\(\displaystyle x=6\)

\(\displaystyle x=-6\)

Correct answer:

\(\displaystyle x=6\)

Explanation:

We need to make the bases equal before attempting to solve for \(\displaystyle x\). Since \(\displaystyle 16=4^2\) we can rewrite our equation as

\(\displaystyle 4^{2x}=4^{2^{2x-6}}\)

    Remember: the exponent rule \(\displaystyle (x^n)^m=x^{n\cdot m}\)

\(\displaystyle 4^{2x}=4^{2(2x-6)}\)

Now that our bases are equal, we can set the exponents equal to each other and solve for \(\displaystyle x\)

\(\displaystyle 2x=2(2x-6)\)

\(\displaystyle 2x=4x-12\)

\(\displaystyle 12=2x\)

\(\displaystyle x=6\)

 

Example Question #1 : Exponential Functions

Solve for \(\displaystyle y\)

\(\displaystyle 7^{2y}-3^{y+2}=0\)

Possible Answers:

\(\displaystyle y=\frac{2\ln3}{2 \ln 7-\ln3}\)

\(\displaystyle y=1\)

\(\displaystyle y=\frac{2\ln3}{2 \ln 7+\ln3}\)

\(\displaystyle y=-1\)

Correct answer:

\(\displaystyle y=\frac{2\ln3}{2 \ln 7-\ln3}\)

Explanation:

The first step is to make sure we don't have a zero on one side which we can easily take care of: 

\(\displaystyle 7^{2y}=3^{y+2}\)

Now we can take the logarithm of both sides using natural log:

\(\displaystyle \ln 7^{2y}=\ln 3^{y+2}\)

Note: we can apply the Power Rule here \(\displaystyle \ln (x^y)=y\ln (x)\)

\(\displaystyle 2y \ln 7=(y+2)\ln3\)

\(\displaystyle 2y \ln 7=y\ln3+2\ln3\)

\(\displaystyle 2y \ln 7-y\ln3=2\ln3\)

\(\displaystyle y(2 \ln 7-\ln3)=2\ln3\)

\(\displaystyle y=\frac{2\ln3}{2 \ln 7-\ln3}\)

Example Question #2 : Solving Exponential Equations

Solve for \(\displaystyle x\)

\(\displaystyle 3e^{(4x-5)}=24\)

Possible Answers:

\(\displaystyle x=\frac{\ln8+5}{4}\)

\(\displaystyle x=\frac{\ln8-5}{4}\)

\(\displaystyle x=\frac{\ln21+5}{4}\)

\(\displaystyle x=\ln2+5}\)

Correct answer:

\(\displaystyle x=\frac{\ln8+5}{4}\)

Explanation:

Before beginning to solve for \(\displaystyle x\), we need \(\displaystyle e\) to have a coefficient of \(\displaystyle 1\)

\(\displaystyle e^{(4x-5)}=8\)

Now we can take the natural log of both sides:

\(\displaystyle \ln e^{(4x-5)}=\ln8\)

Note: \(\displaystyle \ln (e)=1\)

\(\displaystyle 4x-5=\ln8\)

\(\displaystyle 4x=\ln8+5\)

\(\displaystyle x=\frac{\ln8+5}{4}\)

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