a-maths!!!

4 cos y = cosec y

4 cos y = 1/sin y
(4 cos y)(sin y) = 1
(4)(cos y)(sin y) = 1
(2)(2)(cos y)(sin y) = 1

By the formula of (2)(sin y)(cos y)=sin 2y
2 sin 2y =1
sin 2y = 1/2

2y = 30' or 150', 390' or 510'
y = 15' or 75',195' or 255'

2007-12-15 23:00:50 補充：
2y = 30' or 150', 390' or 510'y = 15' or 75',195' or 255'

2007-12-15 23:01:32 補充：
2y = 30' or 150', 390' or 510'y = 15' or 75',195' or 255' 先岩

(i)4cosy=cosecy

π
4cosy = 1 / siny
4cosy - 1 / siny = 0
(4cosysiny - 1) / siny = 0
sin^2 y [(4cosysiny - 1) / siny ] = sin^2 y (0)
siny(4cosysiny - 1) = 0
siny = 0 or (4cosysiny - 1) = 0
y=0 ,π ,2π or 2sin2y - 1 = 0
siny = 1/2
2y =π/6 ,5π/6 , 13π/6 ,17π/6
y = π/12 ,5π/12 ,13π/12 ,17π/12

so y =0 ,π ,2π ,π/12 ,5π/12 ,13π/12 ,17π/12

2007-12-15 22:45:38 補充：
更正:尾4果行係 sin2y = 1/2 唔係siny = 1/2

Solution:

圖片參考：http://hk.geocities.com/stevieg_1023/cos2y.gif