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Seek the second answer in the second stage of compulsory course 5 of senior high school mathematics course guide.
Dddddbaa, 2, 2, -5, root 5,

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