# 6. Exponential and Logarithmic Equations

by M. Bourne

## Solving Exponential Equations using Logarithms

The logarithm laws that we met earlier are particularly useful for solving equations that involve exponents.

### Example 1

Solve the equation `3^x= 12.7`.

### Example 2

Two populations of bacteria are growing at different rates. Their populations at time *t* are given by
`5^(t+2` and
*e*^{2t} respectively. At what time are the
populations the same?

### Exercises

1. Solve `5^x= 0.3`

2. Solve `3\ log(2x − 1) = 1`.

3. Solve for *x*:

`log_2 x + log_2 7 = log_2 21`

4. Solve for *x*:

`3\ ln\ 2+ln(x-1)=ln\ 24`

5. [Reader's question.]

I have the following formula:

`S(n) = 5500\ log\ n + 15000` (Using base 10)

If I know *S*(*n*) = 40 million, How do I solve it?

## Application - World population growth

The population of the earth is growing at approximately `1.3%` per year. The population at the beginning of 2000 was just over `6` billion. After how many more years will the population double to `12` billion?

When the world population is 12 billion, the net number of
people in the world will be increasing at the rate of
about 5 **per second**, if the growth rate is still 1.3%.
Currently, there are about 2.6 new people per second.
However, the rate of growth is expected to drop considerably
to about 0.5% within 50 years.

In 2001, the population of India passed **one billion**, making it the second country after China to reach that scary milestone.

### World population

Current world population is approximately:

**Loading...**

### Interactive applet - World Population

Go to the interactive World Population, which has comparisons between present, past and future population growth.

### Predicting world population

The following graph shows one of the estimates for world population growth during the 21st century. We see that the population will be 11 billion by about 2100! Think of our water quality, air pollution, global warming, social cohesion and lack of food. Surely this is one of the most important graphs in all of mathematics.

But I digress.

We are, of course, talking American English, here. The British billion has 12 zeroes (Well, even they have recently adopted the 9 zeroes billion...).

The world population is expected to exceed 11 billion by 2100. [Source]

This suggests a growth rate of about 0.6%, much lower than that experienced during the 20th century.

The equation for the above graph is

`P=6.1(1.006)^(t-2000)`, where

6.1 billion was the population in 2000;

the growth rate is represented by `1+6/100 = 1.006`; and

`t` is the time from the year 2000.

See a "live" world population estimation on the next page.

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