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