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jverzani
2025-07-27 15:26:00 -04:00
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quarto/precalc/exp_log_functions.qmd
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@@ -22,7 +22,7 @@ The family of exponential functions is used to model growth and decay. The famil
## Exponential functions
The family of exponential functions is defined by $f(x) = a^x, -\infty< x < \infty$ and $a > 0$. For $0 < a < 1$ these functions decay or decrease, for $a > 1$ the functions grow or increase, and if $a=1$ the function is constantly $1$.
The family of exponential functions is defined by $f(x) = a^x, -\infty< x < \infty$ and $a > 0$. For $0 < a < 1$ these functions decay or decrease, for $a > 1$ these functions grow or increase, and if $a=1$ the function is constantly $1$.
For a given $a$, defining $a^n$ for positive integers is straightforward, as it means multiplying $n$ copies of $a.$ From this, for *integer powers*, the key properties of exponents: $a^x \cdot a^y = a^{x+y}$, and $(a^x)^y = a^{x \cdot y}$ are immediate consequences. For example with $x=3$ and $y=2$:
@@ -114,7 +114,7 @@ t2, t8 = 72/2, 72/8
exp(r2*t2), exp(r8*t8)
```
So fairly close - after $72/r$ years the amount is $2.05...$ times more than the initial amount.
So fairly close---after $72/r$ years the amount is $2.05...$ times more than the initial amount.
##### Example
@@ -259,7 +259,7 @@ The inverse function will solve for $x$ in the equation $a^x = y$. The answer, f
That is $a^{\log_a(x)} = x$ for $x > 0$ and $\log_a(a^x) = x$ for all $x$.
To see how a logarithm is mathematically defined will have to wait, though the family of functions - one for each $a>0$ - are implemented in `Julia` through the function `log(a,x)`. There are special cases requiring just one argument: `log(x)` will compute the natural log, base $e$ - the inverse of $f(x) = e^x$; `log2(x)` will compute the log base $2$ - the inverse of $f(x) = 2^x$; and `log10(x)` will compute the log base $10$ - the inverse of $f(x)=10^x$. (Also `log1p` computes an accurate value of $\log(1 + p)$ when $p \approx 0$.)
To see how a logarithm is mathematically defined will have to wait, though the family of functions---one for each $a>0$---are implemented in `Julia` through the function `log(a,x)`. There are special cases requiring just one argument: `log(x)` will compute the natural log, base $e$---the inverse of $f(x) = e^x$; `log2(x)` will compute the log base $2$---the inverse of $f(x) = 2^x$; and `log10(x)` will compute the log base $10$- the inverse of $f(x)=10^x$. (Also `log1p` computes an accurate value of $\log(1 + p)$ when $p \approx 0$.)
To see this in an example, we plot for base $2$ the exponential function $f(x)=2^x$, its inverse, and the logarithm function with base $2$:
@@ -398,7 +398,7 @@ $$
##### Example
Before the ubiquity of electronic calculating devices, the need to compute was still present. Ancient civilizations had abacuses to make addition easier. For multiplication and powers a [slide rule](https://en.wikipedia.org/wiki/Slide_rule) could be used. It is easy to represent addition physically with two straight pieces of wood - just represent a number with a distance and align the two pieces so that the distances are sequentially arranged. To multiply then was as easy: represent the logarithm of a number with a distance then add the logarithms. The sum of the logarithms is the logarithm of the *product* of the original two values. Converting back to a number answers the question. The conversion back and forth is done by simply labeling the wood using a logartithmic scale. The slide rule was [invented](http://tinyurl.com/qytxo3e) soon after Napier's initial publication on the logarithm in 1614.
Before the ubiquity of electronic calculating devices, the need to compute was still present. Ancient civilizations had abacuses to make addition easier. For multiplication and powers a [slide rule](https://en.wikipedia.org/wiki/Slide_rule) could be used. It is easy to represent addition physically with two straight pieces of wood---just represent a number with a distance and align the two pieces so that the distances are sequentially arranged. To multiply then was as easy: represent the logarithm of a number with a distance then add the logarithms. The sum of the logarithms is the logarithm of the *product* of the original two values. Converting back to a number answers the question. The conversion back and forth is done by simply labeling the wood using a logartithmic scale. The slide rule was [invented](http://tinyurl.com/qytxo3e) soon after Napier's initial publication on the logarithm in 1614.
##### Example