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As the title says...

Given a fraction x, what's the terminology for a fraction y, which when added to x equals 1?

I'm assuming there must be a name for this. Something similar 'reciprocal' in concept.

Thank you

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    I would call it *complementary* but with a hint in the text, because the is no standard term (maybe also *complementary to one*). In the context of probabilities, complementary is a perfect fit. – 2017-02-08
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    I don't think there's a standard word for $1-x$... It reminds me _conjugated exponents_ though, but that's not really what is asked. – 2017-02-08
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    Numbers $p, q > 1$ such that $\frac{1}{p}+\frac{1}{q} = 1$ are called Hölder conjugates sometimes. – 2017-02-08
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    Interesting thank you. I'm surprised that there's no common term for it. It's pretty commonly used in e.g. programming. So, just to be clear, is this also the case with percentages and decimal expressions of fractions? (e.g. 30% is .... to 70%). – 2017-02-08
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    If you have $\frac{1}{a}$ then $a$ is it's **multiplicative reciprocal** because $a\cdot \frac{1}{a} = 1$, but that is probably called so because $1$ is the multiplicative identity. Should probably not be used for addition as the additive identity is $0$. – 2017-02-08
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    $1/a+1/b = (a+b)/(ab)$, if equal to 1 then $a+b=ab$ and $a$ and you can say that $b$ are each other's addition-multiplication friends. Result same no matter if you add or multiply them. – 2017-02-08

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