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

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

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.

AB is 6 units

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