1/x = 1/a +1/b [b]
I assume you want to solve this for "b" .....we have.....
1/x = 1/a +1/b subtract 1/a from both sides
1/x - 1a = 1/b get a common denominator on the left
[a - x] / ax = 1/b invert both sdies
[ax] / [ a - x] = b/1
[ax]/ [a - x] = b