Find a common denominator (usually by multiplying the denominators)

Reminder:

Using the method of multiplying the denominators, we multiply the first fraction by the denominator of the second fraction and the second fraction by the denominator of the first fraction.

Remember to multiply both the numerator and the denominator.

For example

Find a common denominator for the fractions: $3 \over 4$ and $2 \over 5$

Solution:

We multiply the fraction $3 \over 4$ by $5$ the denominator of the second fraction and obtain $15 \over 20$

We multiply the fraction $2 \over 5$ by $4$ the denominator of the first fraction and obtain: $8 \over 20$

The common denominator is $20$.

The third step

Sum or subtract numerators only. The denominator will remain the same and will be written once in the final result.

For example

$\frac {3}{13}+\frac {9}{13}=\frac {12}{13}$

Solution:

When the denominator is identical, we will only add the numerators and obtain: $\frac {12}{13}$