Direct-Proportion Edexcel Higher

Direct proportion

What is direct proportion?

• Proportion is a way of talking about how two variables are related to each other

• Direct proportion means that as one variable goes up the other goes up by the same factor

  • The ratio between the two amounts will always stay the same

• The symbol means “proportional to”

  • E.g. y is directly proportional to x, yx

• If x and y are directly proportional, then

  • x : y will always be the same

  • there will be some value, k, such that y = kx

  • the graph relating x and y is a linear graph, with gradient k

• k is called the constant of proportionality

How do I use direct proportion with powers and roots?

• Problems may involve a variable being directly proportional to a power or root of another variable

• For example

  • y is directly proportional to the square of x

    • 

    • means that

  • y is directly proportional to the square root of x

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