#cos y = 1 - 3x^2#
#- sin y \ y' = -6x#
# y' = (6x)/(sin y )#
# = (6x)/(sqrt(1- cos^2 y) )#
# = (6x)/(sqrt(1- (1 - 3x^2)^2) )#
# = (6x)/(sqrt(1- (1 + 9x^4- 6 x^2)) )#
# = (6x)/(sqrt( - 9x^4+ 6 x^2)) #
# = (6x)/(sqrt( 6 x^2 - 9x^4)) #
# = pm (6)/(sqrt( 6 - 9x^2)) #