Solve for e
d = 1/e + 1/f
Move 1/f to the left side of the = sign
d - 1/f = 1/e
Multiply each side by e
e(d - 1/f) = e(1/e)
e(d - 1/f) = 1
Divide out (d - 1/f)
e(d - 1/f) / (d - 1/f) = 1 / (d - 1/f)
e = 1 / (d - 1/f)
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