Question

[brood1989]

Well this is sad but I've got to ask. OG states that
1/k + 1/r is = to r/kr + k/kr (or r + k / kr) For
some reason I'm just not seeing this, can anyone explain
to me what they're doing?

[Brent@GMATPrepNow]

Well this is sad but I've got to ask. OG states that
1/k + 1/r is = to r/kr + k/kr (or r + k / kr) For
some reason I'm just not seeing this, can anyone explain
to me what they're doing?

It's often easier to see what's going on if we use numbers.

For example, to add 1/3 + 1/4, we need a common denominator.
Here the common denominator is 12, so we'll take 1/3 and create an equivalent fraction by multiplying top and bottom by 4 to get 4/12
Similarly, we'll take 1/4 and create an equivalent fraction by multiplying top and bottom by 3 to get 3/12

So, 1/3 + 1/4 is the same as 4/12 + 3/12
Now that we have the same denominators, we can add the numerators to get (4+3)/12 or just 7/12

Now the exact same steps apply to 1/k + 1/r (we still need a common denominator to add these two fractions). Here, the common denominator is kr

So we'll take 1/k and create an equivalent fraction by multiplying top and bottom by r to get r/kr
Similarly, we'll take 1/r and create an equivalent fraction by multiplying top and bottom by r to get r/kr

So, 1/k + 1/r is the same as r/kr + k/kr
Now that we have the same denominators, we can add the numerators to get (r+k)/kr.

[brood1989]

Very clear after your explanation. Thank you very
much.