169 D1.02: Examples 2–5

First, recall how to get an exponent in your calculator. On most calculators, it’s a ^ key or a $y^x$ key.   Find that key and practice using it to evaluate $2^3$. When you can do that correctly, then evaluate the following expression, writing these intermediate values as indicated below.$A=750\cdot {(1+\frac{0.06}{4})}^{4t}=750\cdot {(1+\frac{0.06}{4})}^{4\cdot 5}=750\cdot {\left(1.015\right)}^{20}=750\cdot 1.346855007=1010.141255$

So the investment is worth $1010.14 after 5 years.

It is more convenient to be able to put all of these values into the calculator at the same time rather than writing intermediate steps. We will learn to do that in this course, but that’s not the point of this lesson. The point here is to practice making graphs. This will be useful to you in checking your work when you evaluate formulas. If one of the values you compute doesn’t seem to fit the pattern of the others, that warns you to check your computation again.

We can use the graph to estimate at what time the amount of the investment will be about $4000. Look at the graph to find 4000 on the vertical axis and then notice that the corresponding value on the horizontal axis is about 27.5. So at approximately 27.5 years, the amount of the investment will be $4000.

Check that by plugging it into the formula. $A=750\cdot {(1+\frac{0.06}{4})}^{4\cdot 27.5}=\text{etc.}=3857.68$

This is as close as we could reasonably expect from using a graph to approximate the input value.

If we know how to do some complicated algebra (using logarithms) we can quickly obtain an answer to “how long should we leave the money to earn $4000?” We will not do that particular type of algebra in this course. Instead we will use numerical work to get refine our estimate from the graph. Here, when we look at the graph and the numbers, it is clear that 30 years is too long and 27.5 years is too short. So we try some value in between. Let’s try 28 years.   $A=750\cdot {(1+\frac{0.06}{4})}^{4\cdot 28}=\text{etc.}=3974.28$. This is much closer to the $4000 which was our goal.   If we wanted to try to get it even more accurately, we would use a value larger than 28 years, but only very slightly larger. Since the amount is compounded quarterly, the next time the amount will increase is at 28.25 years, so let’s try that. $A=750\cdot {(1+\frac{0.06}{4})}^{4\cdot 28.25}=4033.89$. That is further away from $4000 than the value for 28 years, so the best answer for this question is that the amount of the investment will be at about $4000 at 28 years.

$A=750\cdot {(1+\frac{r}{n})}^{n\cdot t}=750\cdot {(1+\frac{0.08}{12})}^{12\cdot 5}=750\cdot {\left(1.006666667\right)}^{60}=750\cdot 1.4898457=1117.384281$

So after 5 years, this investment is worth $1117.38.

We can’t graph this easily, because we would need more than a two-dimensional graph. We need one dimension for the output value and one dimension for every input value and investigating four-dimensional graphs is beyond the scope of this course! However, in most practical applications, technicians and scientists isolate one or two input variables that they are most interested in and then analyze the problem with a two-dimensional graph (for one input variable) or a three-dimensional graph (for two input variables.) We will look at some three-dimensional graphs later in the course.

$\begin{array}{}\text{(1)}& & A_t=A_0\cdot {(1+\frac{r}{n})}^{n\cdot t}\text{(2)}& & A_6=900\cdot {(1+\frac{0.07}{2})}^{2\cdot 6}=900\cdot {(1.035)}^{12}=900\cdot 1.511068657=1359.961792\end{array}$

So, after 6 years, this investment is worth $1359.96.