Substitution In Algebra


Method Example
Find the value of
y = x + 2
when x = 3
State Formula y = x + 2
State Substitution x = (3)
Do Substitution y = (3) + 2
Do Calculation y = 5

What is a substitution?
Replacing the letter representing a variable by a number.
Here is an example of substitution:
Area of rectangle = l × w
where l is the length of the longer side
and w is the length of the shorter side.
In a particulat rectangle
we may know that l = 5 and w = 3

State Formula A = l × w
State Substitution l = (5)      w = (3)
Do Substitution A = (5) × (3)
Do Calculation A = 15

Why did you use brackets?
To make things clearer and to reduce the risk of a mistake.
Yes, that was an easy example,
but sometimes we have negative numbers as substitutions and we can make sure that we capture the negative sign inside the brackets so that it does not get lost or used incorrectly.
Also, we may have a more complicated formula to put them in.

Can you show me a more complicated one please?
Certainly, lets do one with three substitutions
where one of them is negative.
Example:
y = m × x + c
find the value of y when m is 2, x is 10 and c is -6

State Formula y = m × x + c
State Substitution m = (2)      x = (10)       c = (-6)
Do Substitution y = (2) × (10) + (-6)
Do Calculation y = 20 + (-6) = 20 - 6 = 14

State Formula

THIS 4 STEP PROCESS IS A GOOD HABIT TO HAVE

[1] You will feel more in control of the work and more confident
[2] It will help you to keep track of where you are in the question
[3] You have a better chance of getting method marks in a question
[4] Better chance of the correct answer and the accuracy marks
[5] It will be easier to check your work

State Substitution
Do Substitution
Do Calculation
 

 
 

 
 
 
 



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