Edexcel IGCSE Maths复习笔记 1.9.1 Compound Interest

Edexcel IGCSE Maths复习笔记 1.9.1 Compound Interest

What is compound interest?

  • Compound interest is where interest is paid on the interest from the year (or whatever time frame is being used) before as well as on the original amount
  • This is different from simple interest where interest is only paid on the original amount

How do you work with compound interest?

  • For COMPOUND changes (can be a decrease as well as an increase):
    • Keep multiplying by the decimal equivalent of the percentage you want


  • Otherwise do the same as normal:
  • Identify “before” & “after” quantities
  • FIND percentage we want:
    • Increase – ADD percentage to 100
    • Decrease – SUBTRACT from 100


  • Write down a STATEMENT connecting “before” and “after”:
    • “after is a percentage of before”


  • Write down the statement as an EQUATION using decimal equivalent
    • remember “is” means “=”



Exam Tip

This method works for any Compound Change – increase or decrease.Remembering "✕ m ✕ m ✕ m = ✕ m3" can make life a lot quicker:It is usually much easier to multiply by decimal equivalent raised to a power than to multiply by the decimal equivalent several times in a row.

Worked Example