aritmetickou
Aritmetickou is the instrumental singular feminine form of the Czech adjective aritmetický, meaning arithmetic. It is used to modify feminine nouns in the instrumental case and does not denote a standalone concept by itself. In Czech, adjectives agree with the nouns they describe in gender, number and case, and aritmetickou appears whenever the noun is feminine and in instrumental.
In practice, aritmetickou commonly appears in mathematical or educational phrases. For example, s aritmetickou operací means
Etymology and relation to arithmetic: the adjective aritmetický derives from aritmetika, the field of arithmetic, itself
See also: aritmetika, aritmetický, Czech grammar, instrumental case.