Exponential Growth & Decay (Edexcel AS Maths: Pure)

What are exponential growth and exponential decay?

• y = Ae^kt is exponential growth

• y = Ae^-kt is exponential decay

 

 

• k > 0 so plus or minus sign in equation immediately indicates growth or decay
• A is the starting value or initial value, when t = 0
• t is often used, rather than x, as many models are time-dependent

Is e always used in exponential growth and decay?

• Using e makes the maths easier
• With the proper choice of k, all exponential functions of the form a^x can be written in the form e^kx
  • a^x = e^kx

 

 

Will logarithms or ln be involved?

 

• ln is the inverse of e

 

 

• The graph of y = ln x is the reflection of the graph of y = e^x in the line y = x