The exponential function
- Ilic Ferretti
NEEDING TO BRUSH UP THE EXPONENTS We have seen that if a quantity changes following an exponential behaviour with respect of a variable , it can expressed by something like: where is the starting amount of (when ) and if the constant factor by which changes. We define exponential function any function where the independent variable appears as exponent, such as Before trying to handle these objects, we need to understand what it means having a power whose exponent can assume any real number. To do this, let's review the meaning of some particular powers we already know. REVIEWING POWERS WITH "SPECIAL" EXPONENTS We all know what a power is; for example means Now let's see what happens when using some unusual exponents. ZERO EXPONENT Using what we know about the properties of powers we can obtain that (if is NOT zero). As a matter of fact we have that can be considered the result of a division like , and this tells us that: where in the last step we simplified all elements both in the numerator and in the denominator: as there is the same number of elements in the numerator and in the denominator, in both parts of the fractions we cancel out everything and we are left with 1. NEGATIVE EXPONENT (exponent is an integer number) Reasoning in a similar way we can obtain that . Once more we use the property of division with like bases, and we consider as the result of a division like . Considering the calculations we have that: When simplifying the elements, we have three elements left in the denominator, because 5 exceeds 2 by 3. This works exactly in the same way of In this case the number of elements in the dividend exceeds the one in the divisor, so there are some elements left in the numerator (i.e. the upper side of the fraction). When the exponent is negative we simply have that there are more elements in the divisor, and therefore some elements remain in the denominator, i.e. in the lower side of the fraction. Please note that in general a negative exponent is equivalent of taking the reciprocal of the base, so FRACTIONARY EXPONENT (exponent is a rational number) Now we will prove that . To obtain that, we must remember the definition of "square root of ": this is the number which, raised to the second, gives us . If the raise to the second we get: This means that behaves exactly as the square root of , and therefore they are the same number. Obviously this works for any fractionary exponent, so we have that as Please note that is the cube root of , as as In general we have that The denominator of the exponent is the index of the root, where the numerator is the usual exponent. IRRATIONAL EXPONENT (exponent is a real number): ??? As we want to consider with being any number, we must ask ourselves what does an irrational exponent means. For example, what's the meaning of ? The answer is not easy, so let's help ourselves with a practical application of exponential functions. We will do it studying the behaviour of waterlily, the nice flower you can see below. In the animation below we will discover that the behaviour of this plant, as supposed in the problem, can be described by an exponential function. We will draw the function giving to the exponent the values we have studied until now. After the animation we will try to go further introducing irrational exponents.
WHAT NEXT? In the example of waterlilies we introduced an exponential function evaluating it for:
- natural exponents, i.e. after a given number of weeks
- negative exponents, which proved to reproduce the behaviour of n weeks ago
- fractionary (rational) numbers, which proved to fit into the curve when evaluating the amount after a non integer number of weeks.
A BRIEF PREVIEW OF LIMIT CONCEPT Let's summarize what we have seen:
- any irrational number con be approximated as precisely as we want with a rational quantity. For instance
- as long as we have a rational number, we know how to consider it as an exponent, so we get an approximation for the value we are looking for. For instance (ok, you probably may find it not very nice, but we do know how to calculate it!)
- to get a better approximation, we simply need to consider more decimals
- so we can "virtually" define as the number obtained considering as many decimals of as we need.
BEHAVIOUR CHANGES IF OR In the waterlilies example we condidered and exponential function with . Its behaviour is similar when assumes many different values, but it is not always the same for any value of . We see this feature in the next animation.
A functions like the exponential with base greater than is called increasing function ("funzione monotona crescente", in Italian), because in any of its parts you have that the greater gets the , the more increases the (i.e the function's result). Another example of increasing function is a line with positive angular coefficient. A functions like the exponential with base less than is called decreasing function ("funzione monotona decrescente", in Italian), because in any of its parts you have that the greater gets the , the more decreases the (i.e the function's result). Another example of increasing function is a line with negative angular coefficient. Finally please note that for the exponential function taking the reciprocal of the base has the same effect of changing the sign of the exponent, that is: the same effect of evaluating the function for instead of . For this reason two exponential functions having reciprocal bases are symmetrical with respect of the axis: considering the first one for a given point gives the same result as considering the other one for (that is the symmetrical of with respect of the y axis). You can verify this feature drawing point by point two such functions, like the ones used in previous animation or and .