tg a : ctg a= tg a
sin a/cos a : cos a/sin a = sin a/cos a
sin a/cos a * sin a/cos a - sin a/cos a = 0
sin^2 a/cos^2 a - sin a/cos a = 0
(sin^2 a - sin a * cos a)/ cos^2 a =0
( sin a(sin a-cos a))/ cos^2 a = 0
Или
tg a : ctg a= tg a
tg a : 1/tg a = tg a
tg a * tg a = tg a
tg^2 a -tg a = 0
tg a( tg a -1) = 0