exponential function. y = a x if we look at the graphs of y = a x for different values of a, we can...
TRANSCRIPT
Exponential Function
y = ax
If we look at the graphs of y = ax for different values of a, we can see that the gradient of the exponential graph increases as the value of a increases
a gradient
y = ax
a increasing
• Gradient of y = 2x at (0,1) is 0.693
• Gradient of y = 3x at (0,1) is 1.099
• So there must be a value (between 2 and 3) where the gradient at (0,1) is exactly 1
• This number is 2.71828… and is called e
Natural Logarithms
By drawing the graph of y = ex we can see that there is an inverse function
If y = ex
Take logs of both sides logey = logeex
logey = xlogee but logee = 1
So x = logey
Interchanging x and y gives y = logex
(written as y = lnx)
This is the inverse function
Questions
1. Make x the subject of lnx – lnA = kt
2. Make t the subject of s = e-kt
3. Make x the subject of y-5 = (A - 5)ex
Questions4. A colony of human settler on a previously
uninhabited planet. After t years, their population, P, is given by
P = 100e0.056t
a. Sketch the graph of P against t b. How many settlers were there planet
initially? c. How long does it take for the
population to reach 1 million?
Answers1. lnx – lnA = kt ln(x/A) = kt
x/A = ekt
x = Aekt
2. s = e-kt lns =-kt t =-lns/k
3. y-5 = (A - 5)ex ex =(y-5)/(A - 5) x =ln((y-5)/(A - 5))
Answers ctd4. P = 100e0.056t
b. When t= 0 P = 100e0 = 100
c. P = 100e0.056t = 1,000,000 e0.056t = 10,000
0.056t = ln 10,000= 9.2103 t = 9.2103/0.056 = 164.47
.47 years =12x.47 months = 5.64monthsIt takes 164.5 years to 4 s.f. or 164 years 6 months to the nearest month