Evaluate 2/a+1/b=13/15

Finding the LCD of a list of values is the same as finding the LCM of the denominators of those values.

Since ${\displaystyle a,15,b}$ contain both numbers and variables, there are two steps to find the LCM. Find LCM for the numeric part ${\displaystyle 1,15,1}$ then find LCM for the variable part ${\displaystyle a^1,b^1}$.

The LCM is the smallest positive number that all of the numbers divide into evenly.

1. List the prime factors of each number.

2. Multiply each factor the greatest number of times it occurs in either number.

The number ${\displaystyle 1}$ is not a prime number because it only has one positive factor, which is itself.

Not prime

${\displaystyle 15}$ has factors of ${\displaystyle 3}$ and ${\displaystyle 5}$.

The number ${\displaystyle 1}$ is not a prime number because it only has one positive factor, which is itself.

Not prime

The LCM of ${\displaystyle 1,15,1}$ is the result of multiplying all prime factors the greatest number of times they occur in either number.

Multiply ${\displaystyle 3}$ by ${\displaystyle 5}$.

The factor for ${\displaystyle a^1}$ is ${\displaystyle a}$ itself.

${\displaystyle a}$ occurs ${\displaystyle 1}$ time.

The factor for ${\displaystyle b^1}$ is ${\displaystyle b}$ itself.

${\displaystyle b}$ occurs ${\displaystyle 1}$ time.

The LCM of ${\displaystyle a^1,b^1}$ is the result of multiplying all prime factors the greatest number of times they occur in either term.

Multiply ${\displaystyle a}$ by ${\displaystyle b}$.

The LCM for ${\displaystyle a,15,b}$ is the numeric part ${\displaystyle 15}$ multiplied by the variable part.

Multiply each term in ${\displaystyle \frac{2}{a}=\frac{13}{15}-\frac{1}{b}}$ by ${\displaystyle 15ab}$.

Simplify the left side.

Simplify the right side.

Rewrite the equation as ${\displaystyle 13ab-15a=30b}$.

Factor ${\displaystyle a}$ out of ${\displaystyle 13ab-15a}$.

Divide each term in ${\displaystyle a(13b-15)=30b}$ by ${\displaystyle 13b-15}$ and simplify.