Crossword Genius

In mathematics, an angle of less than 90 (5)

Ross

I believe the answer is:

acute

(Other definitions for acute that I've seen before include "Discerning" , "Severe, penetrating" , "(Of an angle) less than 90 degrees" , "Shrewd; serious" , "Angle of less than 39 degrees" .)

I've seen this clue in The Mirror.
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