Why must the excluded values of each expression in a sum or difference of rational expressions also be excluded values for the simplified expression

If included, they will make the denominator in the simplified expression equal to zero.

Step-by-step explanation:

-Excluded values are values that will make the denominator of a fraction equal to 0. You can't divide by 0, so it's very important to find these excluded values when you're solving a rational expression.

-They would have a similar effect to the simplified expression, will make the denominator of a fraction equal to 0.

-Hence, we exclude them in the simplified expression too.