# GRE Subject Test: Math : Exponential Functions

## Example Questions

### Example Question #21 : Exponential Functions

Solve for

Explanation:

Step 1: Rewrite the right side as a power of :

Step 2: Rewrite the original equation:

Step 3: Since the bases are equal, I can set the exponents equal.

So,

### Example Question #21 : Exponential Functions

Solve for

Explanation:

Rewrite the right hand side such that it has the same base as the expression on the right hand side.

Apply the rule of stacked exponents. We multiply the exponents together and keep the same base:

Now that we have rewrote the right hand side in the same way as the left, we can equate them:

Now that we have the same bases, we can equal the exponents and solve for x...

### Example Question #22 : Exponential Functions

Find the value  that makes the statement  true.

Explanation:

Step 1: Re-write the left side so we have the same base as the right side...

Step 2: Multiply the stacked exponents on the left hand side...

We will apply the rule:

So,

Step 3: We can now set the exponents equal to each other, since we have the same base.

### Example Question #21 : Exponential Functions

Find one possible value of , given the following equation:

Cannot be determined from the information given.

Explanation:

We begin with the following:

This can be rewritten as

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:

Solve this to get t.

### Example Question #1 : Exponential Growth Applications

Suppose a blood cell increases proportionally to the present amount.  If there were  blood cells to begin with, and  blood cells are present after  hours, what is the growth constant?

Explanation:

The population size  after some time  is given by:

where  is the initial population.

At the start, there were 30 blood cells.

Substitute this value into the given formula.

After 2 hours, 45 blood cells were present.  Write this in mathematical form.

Substitute this into , and solve for .

### Example Question #1 : Constant Of Proportionality

Suppose a population of bacteria increases from  to  in . What is the constant of growth?

None of these

Explanation:

The equation for population growth is given by .  is the population,  is the intial value,  is time, and  is the growth constant. We can plug in the values we know at time  and solve for  .

Now that we solved for , we can plug in what we know for time  and solve for .

### Example Question #6 : Constant Of Proportionality

A population of deer grew from 50 to 200 in 7 years. What is the growth constant for this population?

None of these

Explanation:

The equation for population growth is given by . P is the population,  is the intial value,  is time, and  is the growth constant. We can plug in the values we know at time  and solve for  .

Now that we have solved for  we can solve for  at

### Example Question #7 : Constant Of Proportionality

A population of mice has 200 mice. After 6 weeks, there are 1600 mice in the population. What is the constant of growth?

Explanation:

The equation for population growth is given by .  is the population,  is the intial value,  is time, and  is the growth constant. We can plug in the values we know at time  and solve for .

Now that we have  we can solve for  at .

### Example Question #1 : Constant Of Proportionality

The rate of decrease of the dwindling wolf population of Zion National Park is proportional to the population. The population decreased by 7 percent between 2009 and 2011. What is the constant of proportionality?

Explanation:

We're told that the rate of growth of the population is proportional to the population itself, meaning that this problem deals with exponential growth/decay. The population can be modeled thusly:

Where  is an initial population value, and  is the constant of proportionality.

Since the population decreased by 7 percent between 2009 and 2011, we can solve for this constant of proportionality:

### Example Question #1 : Constant Of Proportionality

The rate of growth of the Martian Transgalactic Constituency is proportional to the population. The population increased by 23 percent between 2530 and 2534 AD. What is the constant of proportionality?