Question

Which of the following can be added without altering any of the fractions?

Answer 1

Answer: Choice A

Why? Because the denominators are both the same at 2x+8. We can only add or subtract fractions when the denominators are the same. The general rule for adding fractions is

$\frac{A}{C}+\frac{B}{C} = \frac{A+B}{C}$

We have numerators A and B added together over a common denominator C. If we didn't have a common denominator, then we'd have to do a bit of algebraic manipulation to get both fractions to a common denominator. It doesn't have to be the lowest common denominator (LCD), though it's often more efficient that way.