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4. Exponential and logarithmic functions
4.1 Exponential Functions
A function of the form f(x) = ax, a > 0 , a 1 is called an exponential function. Its domain is the set of all real
numbers. For an exponential function f we have a f x
f x
. The graph of an exponential function depends
on the value of a. a> 1 0 < a< 1
Points on the graph: (-1, 1/a), (0,1), (1, a)
Properties of exponential functions
- The domain is the set of all real numbers: Df = R
- The range is the set of positive numbers: Rf = (0, +). (This means that ax^ is always positive , that is ax^ > 0 for all x. The equation ax^ = negative number has no solution)
- There are no x-intercepts
- The y-intercept is (0, 1)
- The x-axis (line y = 0) is a horizontal asymptote
- An exponential function is increasing when a > 1 and decreasing when 0 < a < 1
- An exponential function is one to one, and therefore has the inverse. The inverse of the exponential function f(x) = ax^ is a logarithmic function g(x) = loga(x)
- Since an exponential function is one to one we have the following property: If au^ = av^ , then u = v. (This property is used when solving exponential equations that could be rewritten in the form au^ = av.)
Natural exponential function is the function f(x) = ex, where e is an irrational number, e 2.718281….
The number e is defined as the number to which the expression ( 1 n^1 ) n approaches as n becomes larger and
larger. Since e > 1, the graph of the natural exponential function is as below
-5 -4 -3 -2 -1 1 2 3 4 5
1
2
3
4
5
x
y
(-1, 1/a) (1,a) -5 -4 -3 -2 -1 1 2 3 4 5
1
2
3
4
5
x
y
(-1, 1/a) (1,a)
Example: Use transformations to graph f(x) = 3 -x^ - 2. Start with a basic function and use one transformation at a time. Show all intermediate graphs. This function is obtained from the graph of y = 3x^ by first reflecting it about y-axis (obtaining y = 3-x) and then shifting the graph down by 2 units. Make sure to plot the three points on the graph of the basic function! Remark : Function y = 3x^ has a horizontal asymptote, so remember to shift it too when performing shift up/down y = 3x^ y = 3 – x^ y = 3 – x^ - 2
Example: Use transformations to graph f(x) = 3e2x-1. Start with a basic function and use one transformation at a time. Show all intermediate graphs.
Basic function: y = ex^ y = ex-1^ (shift to the right by1)
-5 -4 -3 -2 -1 1 2 3 4 5
1
2
3
4
5
x
y
(-1, 1/e)
(1,e)
c) log 2 (3) is an exponent to which 2 must be raised to obtain 3: 2x^ = 3. We know that such a number x exists, since 3 is in the range of the exponential function y = 2x^ (there is a point with y-coordinate 3 on the graph of this function) but we are not able to find it using traditional methods. If we want to refer to this number, we use log 2 (3).
The relationship in (I) allows us to move from exponent to logarithm and vice versa Example:
- Change the given logarithmic expression into exponential form: log 2 x = 4 The exponential form is: 2^4 = x. Notice that this process allowed us to find value of x, or to solve the equation log 2 (x) = 4
- Change the given exponential form to the logarithmic one: 2x^ = 3. Since x is the exponent to which 2 is raised to get 3, we have x = log 2 (3). Note that the base of the exponent is always the same as the base of the logarithm.
Common logarithm is the logarithm with the base 10. Customarily, the base 10 is omitted when writing this logarithm: log 10 (x) = log(x) Natural logarithm is the logarithm with the base e (the inverse of y = ex): ln(x) = loge(x)
(II) Domain of a logarithmic function = (0, ) (We can take a logarithm of a positive number only.) Range of a logarithmic function = (-, + ) (III) loga(ax) = x, for all real numbers a log a^ ( x ) x , for all x > 0
Example log 225 = 5, lne^3 = 3, 3 log 3 (^2 ) 2 , eln7^ = 7
(IV) Graph of f(x) = loga(x) is symmetric to the graph of y = ax^ about the line y= x
a > 1 0 < a < 1
Points on the graph of y = loga(x) : (1/a, -1), (1,0), (a, 1)
(V) The x-intercept is (1, 0). (VI) There is no y-intercept (VII) The y-axis (the line x = 0) is the vertical asymptote (VIII) A logarithmic function is increasing when a > 1 and decreasing when 0 < a < 1 (IX) A logarithmic function is one to one. Its inverse is the exponential function
-5 -4 -3 -2 -1 1 2 3 4 5
1
2
3
4
5
x
y
(-1, 1/a)
(1,a)
y = ax y=x
(1/a, -1)
(a,1) (^) y = loga(x)
-5 -4 -3 -2 -1 1 2 3 4 5
1
2
3
4
5
x
y
(-1, 1/a) (1,a)
y = ax y=x
(1/a, -1)
(a,1)
y = loga(x)
(X) Because a logarithmic function is one to one we have the following property: If loga(u) = loga(v), then u = v (This property is used to solve logarithmic equations that can be rewritten in the form loga(u) = loga(v).)
Example: Use transformations to graph f(x) = -2log 3 (x-1) + 3. Start with a basic function and use one transformation at a time. Show all intermediate graphs. Plot the three points on the graph of the basic function
a) y = log 3 (x) b) y = log 3 (x-1) c) y = 2log 3 (x-1)
d) y =-2log 3 (x-1) e) y = -2log 3 (x-1) + 3
Remark: Since a logarithmic function has a vertical asymptote, do not forget to shift it when shifting left/right
Example: Find the domain of the following functions (A logarithm is defined only for positive (> 0) values)
a) f(x) = log1/2(x^2 – 3) Df: x^2 – 3 > 0 x^2 – 3 = 0 x^2 = 3 x= 3
b) e-2x+1^ = 13 This is an exponential equation that can be solved by changing it to the logarithmic form -2x+ 1 = loge(13) -2x+1 = ln(13) -2x= -1 + ln x = 2
1 ln 13 2
1 ln (^13)
Since this is an exponential equations, there are no restrictions on x. Solution is x = 2
1 ln 13
4.3 Properties of logarithms Properties of logarithms:
Suppose a > 0, a 1 and M, N > 0
(i) loga(1) = 0 loga(a) = 1 Example: log 2 (1) = 0 log 15 (15)= 1 ln(1) = 0 ln(e) = 1
(ii) a^ M
log a ( M ) Example: 6 log^6 (^7 ) 7 eln(4)^ = 4
(iii) loga(ar) = r Example: log 3 (3^4 ) = 4 ln(e2x) = 2x
(iv) loga(MN) = loga(M) + loga (N) Example : log 5 (10)= log 5 (5)+log 5 (2) loga(M) + loga (N) = loga(MN) ln(x+1) + ln(x-1)= ln[(x+1)(x-1)]
(v) Example:
log log ( M ) log ( N ) N
M a a a
log( 15 ) log( 2 ) 2
15 log (^4) 4 4
N
M log a ( M ) log a ( N ) log a
3
12 log 4 ( 12 ) log 4 ( 3 ) log 4
(vi) loga(Mr)= rloga(M) Example: log(3x) = xlog(3) rloga(M) = loga(Mr) 5log 3 (x+1)= log 3 [(x+1)^5 ]
(vii) If M = N, then loga(M) = loga(N) Example: if x = 4, then loga(x) = loga(4) If loga(M) = loga(N), then M = N if log 4 (x-1) = log 4 (2x-5), then x-1 = 2x-
(viii) Change of the base formula
log ( )
log log ( ) a
M M b
b a , where b is any positive number different than 1
In particular,
log( )
log log ( ) a
M a M ^ and
ln( )
ln log ( ) a
M a M This formula is used to find values of logarithms using a calculator.
Example: Evaluate log 2 (3)
- 5849 ln( 2 )
ln 3 log 2 ( 3 )
Example : Write
1
( 2 ) log (^2)
3 3 x
xx as a sum/difference of logarithms. Express powers as product.
log( ) 3 log( 2 ) log( 1 )
log( ) log[( 2 )] log 1
log[ ( 2 )] log 1 1
log (^2 )
2 2 3 1 3 3
2 1 /^2 3
3 3 3
2 3
3 2 3
3 3
x x x
x x x
xx x x
xx
Example: Write as a single logarithm
3log 4 (3x+1) – 2log 4 (2x-1)- log 4 (x) = = log 4 [(3x+1)^3 ] – log 4 [(2x-1)^2 ] – log 4 (x)=
2
3 4
2
3
2 4 4
3 (^4) ( 2 1 )
( 3 1 ) ( 2 1 ) log
( 3 1 )
log( ) log ( 2 1 )
( 3 1 ) log x x
x x
x
x
x x
x
4.4 Exponential and logarithmic equations
A logarithmic equation is an equation that contains a variable “ inside “ a logarithm. Since a logarithm is defined only for positive numbers, before solving a logarithmic equation you must find its domain ( alternatively, you can check the apparent solutions by plugging them into the original equation and checking whether all logarithms are well defined).
There are two types of logarithmic equations: (A) Equations reducible to the form loga(u) = r, where u is an expression that contains a variable and r is a real number
To solve such equation change it to the exponential form ar^ = u and solve.
Example: Solve 3log 2 (x-1)+ log 2 (3) = 5
(i) Determine the domain of the equation. (What is “inside” of any logarithm must be positive) x-1 > 0 x > 1 (Only numbers greater than 1 can be solutions of this equation)
(ii) Use properties of logarithms to write the left hand side as a single logarithm log 2 (x-1)^3 + log 2 (3) = 5 log 2 (3(x-1)^3 ) = 5
(iii) Change to the exponential form 25 = 3(x-1)^3
To solve such equation, change into logarithmic form and solve Example : Solve 3 4 2x-1^ = 5 (i) Write the equation in the desired form (exponent = a number) 4 2x-1^ = 5/ (ii) Change to the logarithmic form 2x-1 = log 4 (5/3) (iii) Solve 2x = 1 + log 4 (5/3) x = 2
1 log 4 ( 5 / 3 )
To find an approximate value, use the change of the base formula to rewrite log 4 (5/3) as log(5/3)/log
(B) Equations that can be reduced to the form au^ = av.
To solve such an equation use the property of exponential functions that says that if au^ = av, then u = v and solve it.
Example Solve 16 2 46
2 x x (i) Use the properties of exponents to write the equation in the desired form. Notice that all bases (16, 2, 4) are powers of 2, 16 = 2^4 , 2 = 2^1 , 4 =2^2.
2 x x
4 12
4 12
4 26
2 2
2 2 2
2 2 2
2
2
2
x x
x x
x x
(ii) Use the property (7) 4x + x^2 = 12
(iii) Solve x^2 + 4x – 12 = 0 (x+6)(x-2) = 0 x = - 6 or x = 2
Solutions: -6, 2
(C) Equations that can be reduced to the form au^ = b v
To solve such equation apply the log (or ln ) to both sides of the equation (property (vii) of logarithms), use the property of logarithms to bring the u and v outside of the logarithms and solve for the variable. Keep in mind that log(a) and log(b) are just numbers ( like 1.34 or 3)
Example: Solve 2x+1^ = 51-2x (i) Apply log to both sides log(2x+1) = log(51-2x) (ii) Use properties of logarithms. (Enclose the powers into the parentheses) (x+1)log(2) = (1-2x)log(5) (iii) Solve
Eliminate parentheses xlog(2)+ log(2) = log(5) -2xlog(5)
Bring the terms with x to the left hand side x log(2) + 2xlog(5) = log(5) – log(2)
Factor out x x(log(2)+2log(5)) = log(5) – log(2) Divide, to find x x = log( 2 ) 2 log( 5 )
log( 5 ) log( 2 )
You could use properties of logarithms to write the solution as x = log( 50 )
log( 5 / 2 ) log( 2 5 )
log( 5 / 2 ) ^2
If an exponential equation cannot be transformed to one of the types above, try to substitute by u an exponential expression within the equation. This might reduce the equation to an algebraic one, like quadratic or rational.
Example: Solve 22x^ + 2x+2^ -12 = 0
(i) Rewrite the equation so that 2x^ appears explicitly (2x)^2 + 2x^ 22 – 12 = 0 (2x)^2 + 4(2x) – 12 = 0 (ii) Substitute u = 2x u^2 + 4u – 12 = 0 (iii) Solve the equation for u (u+6)(u-2) = 0 u = -6 or u = 2 (iv) Back- substitute and solve for x 2 x^ = -6 or 2 x^ = 2 No solution x = 1 Solution: x = 1
