**Least Common Denominator**

- Before we solve problems using fractions we must first have a
**L**east**C**ommon**D**enominator (LCD). - If the
**L**east**C**ommon**D**enominator (also sometimes called**L**east**C**ommon**M**ultiple) is not obvious, we can always find the unit by finding the shortest rod that is a multiple of each denominator.

**For Example:**

In order to find the **L**east **C**ommon **D**enominator
for the fractions **1/2** and **1/3**
we must first choose rods which represent both of the denominators. In
this case the denominator **2** would be represented by the red rod
and the denominator **3** would be represented by the light green rod.
Now create a **train **(rods lined up end-to-end) of red rods and a
train of light green rods until they are equal in length. (These trains
are also known as multiples). Once you have created trains equal in length,
total the number of centimeters in each train. As shown below 3 red rods
= 6 cm. and 2 light green rods = 6 cm. Therefore, the common denominator
is **6**.

**Let's try another example:**

Find the least common denominator for the fractions **1/4**
and **2/3**. The denominator **4** will
be represented by the purple rod and the **3** will be represented by
the light green rod. Now we make our trains.

As shown above **3 **purple rods = 12 cm. and **4** light green
rods = 12 cm.

**Now you try to find the Least Common Denominator using your Cuisenaire
Rods.**

**1.** **1/6 and 2/3**

**2. 5/6 _{ }and 3/4**

**3. _{ }1/8_{ }and 1/2 **

**Check your
Answers.**

Send comments to: silha@girltech.cs.rice.edu

**This page was developed through GirlTECH
'97, a teacher training and student technology council program sponsored
by the Center for Research on
Parallel Computation (CRPC), a National
Science Foundation-funded Science
and Technology Center. **

**© June 1997 Molly Silha **