Pada gambar tolong dibantu soal trigo

BD = BC tan 20
BD = 50 tan 20
sin 40 = BD/AD
AD = BD/sin 40
AD = (50 tan 20)/sin 40
AD = (50 tan 20)/2sin 20.cos 20
AD = (25 tan 20)/ sin 20.cos 20
AD = 25( sin 20/ cos 20 ) / sin 20.cos 20
AD = 25/cos^2 .20
AD = 25(sec 20°)^2.....E.