Two equations that have the same solution are called equivalent
equations e.g. 5 +3 = 2 + 6. And this as we learned in a previous
section is shown by the equality sign =. An inverse operation are
two operations that undo each other e.g. addition and subtraction
or multiplication and division. You can perform the same inverse
operation on each side of an equivalent equation without changing
This gives us a couple of properties that hold true for all
The addition property of equality tells us that adding
the same number to each side of an equation gives us an equivalent
The same goes with the subtraction property of
As well as it goes for the multiplication property of
equality. If you multiply each side of an equation with the
same nonzero number you produce an equivalent equation.
And naturally this goes for the division property of
equality as well. You can divide each side of an equation with
the same nonzero number to produce an equivalent equation
This gives us a way to change an equation to our own liking.
Anything is acceptable as long as you do the same thing on both
There are a couple of other properties of equations that can be
good to know as well
George has cut down a 60ft high oak tree. He now wants to chop
it into smaller pieces. He first cuts it into two pieces that are
both 30ft. And then he continues on into making ten pieces that all
are 6ft long before loading them onto his truck.
By looking at the different pieces of wood we can see that the
following holds true.
This is called the reflexive property of equality and
tells us that any quantity is equal to itself
We can also use this example with the pieces of wood to explain
the symmetric property of equality. This property states
that if quantity a equals quantity b, then b equals a.
Or if we use our example
Another property that can be explained by this is the
transitive property of equality. It tells us that if a
quantity a equals quantity b, and b equals the quantity, c, then a
and c are equal as well.
Or in the numbers taken from the oak tree example
Since we know that 30 + 30 = 20 + 40 and that 30 + 30 = 60 we
can substitute 30 + 30 for 20 + 40 and get 60 = 20 + 40. This is
called the substitution property of equality.
If a = b, then a can be substituted for b in any expression.
Videolesson: Solve these equations using