Englishfor English speakers
reciprocity
Noun
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Reciprocity is a relationship in which a something is given, and something is expected back in return.
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Reciprocity is a relationship which means quid pro quo or "fair trade".
You scratch my back, and I'll scratch your back: that's reciprocity.
formula
Noun
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A formula is a way to do something or solve a problem that does not change much.
There is no formula for making profit, yet many business people think there is.
The formula for converting from kilograms to pounds is mathx\ \mathrm{kg} \times 2.2/math
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Infant formula is a powder that you mix with water to make a drink for babies.