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Substitution Property of Equality
See It in Action
Two examples of the idea done right—and one common look-alike that is not right.
EXAMPLE
EXAMPLE
NON-EXAMPLE
Definition
The Substitution Property of Equality in Geometry states:
“If two values are equal, then one can be substituted for the other in any expression or equation.”
In other words:
If a = b, then you can replace a with b (or vice versa) in any mathematical statement without changing its truth.
Example:
If you know that
x = 5
and
y = x + 3,
then you can substitute 5 for x and write:
y = 5 + 3 = 8
Why it matters in Geometry:
The Substitution Property helps when working with congruent segments or angles. If you know two things are equal (or congruent), you can swap them in a proof or calculation.
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