substitutions

Algebra - Substitution

"Substitute" means to put in the place of another.

Substitution

In Algebra "Substitution" means putting numbers where the letters are:

	If you have:	x - 2
	And you know that x=6 ...
	... then you can "substitute" 6 for x:	6 - 2 = 4

Example 1: If x=5 then what is 10/x + 4 ?

Put "5" where "x" is:

10/5 + 4 = 2 + 4 = 6

Example 2: If x=3 and y=4, then what is x² + xy ?

Put "3" where "x" is, and "4" where "y" is:

3² + 3×4 = 9 + 12 = 21

Example 3: If x=3 (but you don't know "y"), then what is x² + xy ?

Put "3" where "x" is:

3² + 3y = 9 + 3y

(that is as far as you can get)

As that last example showed, you may not always get a number for an answer, sometimes just a simpler formula.

Negative Numbers

When substituting negative numbers, put () around them so you get the calculations right.

Example 4: If x = -2, then what is 1-x+x² ?

Put "(-2)" where "x" is:

1 - (-2) + (-2)² = 1 + 2 + 4 = 7

Note: if you don't know why

• the - (-2) became +2 or
• the (-2)² became +4

then here is a quick summary:

	Rule	Adding or Subtracting	Multiplying or Dividing
	Two like signs become a positive sign	3+(+2) = 3 + 2 = 5	3 × 2 = 6
		6-(-3) = 6 + 3 = 9	(-3) × (-2) = 6
	Two unlike signs become a negative sign	7+(-2) = 7 - 2 = 5	3 × (-2) = -6
		8-(+2) = 8 - 2 = 6	(-3) × 2 = -6