Edexcel IGCSE Maths 复习笔记 6.6.1 Combined Probability - Basics

Edexcel IGCSE Maths 复习笔记 6.6.1 Combined Probability - Basics

What do we mean by combined probabilities?

  • This can mean lots of things as you’ll see over these notes and the next set
  • In general it means we have more than one ‘thing’ (trial/event) to bear in mind and these things may be independentmutually exclusive or may involve an event that follows on from a previous event drawing a second counter from a bag



1. Tree diagrams

  • Especially useful when we have more than one trial but are only concerned with two outcomes from each
  • Even more useful when probabilities change for the second experiment


2. Replacement

  • Are items being selected at random replaced or not?
  • If not then numbers will decrease as the situation progresses and so probabilities change – this is often called conditional probability


3. AND’s and OR’s

  • AND means for independent events
  • OR means for mutually exclusive events


4. Sum of all probabilities is 1

  • This is a very basic fact that gets lost along the way in more complicated probability questions – but it is one of the best ‘tricks’ you can use!
  • A good example of its use is when you want the probability of something being “non zero”:P(x ≥ 1) = 1 - P(x = 0)



Worked Example


Worked Example


Worked Example


Worked Example