sin40º(1+2cos40º)／(2cos²40º+cos40º-1)
=(sin40º+2sin40ºcos40º)／(2cos²40º-1+cos40º)
=(sin40º+sin80º)/(cos40º+cos80º)
=[2sin(40+80)/2 *cos(40-80)/2]/[2cos(80+40)/2 *cos(80-40)/2]
=2sin60ºcos20º/(2cos60ºcos20º)
=tan60º
=√3
cos2α = 2cos^2 α- 1= 1 − 2sin^2 α= cos^2 α − sin^2 α
sin2a = 2sinacosa
tan2α = 2tanα/[1 - (tanα)^2]