Historical clue formulations

• Inverse function in trigonometry — 2017-10-20
• Inverse trig function — 2015-11-15
• Trig inverse — 2012-04-29
• 30°, for 0.5 — 2011-07-08
• Trig angle — 2008-09-21
• Function whose domain is between -1 and 1 — 2008-03-01
• Inverse math function — 1999-08-03
• It's measured in radians — 1999-01-23