15 proves that:
c(cosB/b-cosA/a)
=c{[(a^2+c^2-b^2)/2ac]/b-[(b^2+c^2-a^2)/2bc]/a}
=(a^2+c^2-b^2)/2ab-(b^2+c^2-a^2)/2ab
=(2a^2-2b^2)/2ab
=(a^2-b^2)/ab
=a/b-b/a
So a/b-b/a=c(cosB/b-cosA/a)
16 if the vertical intersection d is CF intersection m and CF intersection n, then
DM=50,EM= 120,MN= 120,NF=30
According to Pythagorean theorem, DE = 130, DF = 10 √ 298, EF = 150.
Derived from cosine theorem
cos∠DEF= 130? + 150? -( 10√298)? /(2× 130× 150)= 16/65