Trig function that becomes another trig function by adding "co" to the beginning (4)

I believe the answer is:

sine

(Other definitions for sine that I've seen before include "Latin tag for indefinitely - . . . . die" , "Indefinitely, with no appointed date - . . . . die" , "Trig function" , "Trigonometrical function" , "Without; maths function" .)

I've seen this clue in The Washington Post.

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