the angle ACD=0; AD=a. Then sn BDF =n.sin EDC; or 
= sin 0 = pes sin 0; or BC.ED=n. BD. EC. 
payee edinley Re. eal, 
SISTA) Cyrene eh Gers Ml Cates 
(neglecting in the expression for cos @ the powers above the second) 
flea MN Net «3 SHI RDP S TiN ChE 1 z a 
Stee bea A= Ht pep a7 Dt CTP) 5 
3's 
since a = AC x 0 = 
Af a 1 a 
Similarly, ED = ae) 
Hence the equation becomes 
Crip Getta) =A a) Gy taD-5)- 
or (r+D) (1-2.7=2.£) =n (r—2) CEES: 
r+D n—1 D 
= > and «= rT——. 
n n n 
Let a=0; r+ D=n(r—-x2); r—x 
2 
Substituting these approximate values in the coefficient of =: 
(r+D) €é - =. (r+D) (n=1 .7—D).£) =n (7-2) ( +D (D+r)£), 
a? (r+D) (n—1 r—D)+(r+D).D.n*) 
2 me J 
r+D 
n 
hence r—2 = {1 = 
