Does sine 40 equal sin 20?

NO!!!

Sine40 = Cosine50 = 0.64278...

Sin30 = Cos 60 = 0.5

Sin 0 = Cos 90 = 0

Sin 90 = Cos 0 = 1

From Latin the Word 'Sine' means 'curve'. Cosine means 'Complimentary Sine' or Complimentary curve'.

The 'awkward' decimal numbers are based on a the radius of a circle, that radius being the hypotenuse of a right- triangle based at the circles origin/centre.