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Answered 2013-04-25 00:14:49

If you have:x-2And 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 = 3×3 + 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 NumbersWhen 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: RuleAdding or SubtractingMultiplying or DividingTwo like signs become a positive sign3+(+2) = 3 + 2 = 53 × 2 = 66-(-3) = 6 + 3 = 9(-3) × (-2) = 6Two unlike signs become a negative sign7+(-2) = 7 - 2 = 53 × (-2) = -6 (-3) × 2 = -68-(+2) = 8 - 2 = 6

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