If the two rational expressions that you want to add or subtract have the same denominator you just add/subtract the numerators which each other.
Example
xx−1+2−xx−1=x+2−xx−1=2x−1
When the denominators are not the same in all expressions that you want to add or subtract as in the example below you have to find a common denominator. The easiest way to do this is to multiply the denominators with each other, but that might not get the simplest computations and usually requires a lot of simplifying afterwards, but it's a method that always works if you're uncertain. A way to get the usually easiest computations is to find the least common denominator (LCD). The LCD is the least number that is a common multiple of the two or more numbers in the denominator.
Answers & Comments
Answer:
If the two rational expressions that you want to add or subtract have the same denominator you just add/subtract the numerators which each other.
Example
xx−1+2−xx−1=x+2−xx−1=2x−1
When the denominators are not the same in all expressions that you want to add or subtract as in the example below you have to find a common denominator. The easiest way to do this is to multiply the denominators with each other, but that might not get the simplest computations and usually requires a lot of simplifying afterwards, but it's a method that always works if you're uncertain. A way to get the usually easiest computations is to find the least common denominator (LCD). The LCD is the least number that is a common multiple of the two or more numbers in the denominator.