Tính C1cosa+1=1+cosacosa=2cos2a2cosa
1cos2a+1=1+cos2acos2a=2cos2acos2a
...
1cos2na+1=1+cos2nacos2na=2cos22n−1acos2na
⇒C=2n+1.cos2a2.cosa.cos2a......cos2n−1a.1cos2na
=2n+1.cosa2.1cos2na.(cosa2.cosa......cos2n−1a)
Áp dụng công thức tính B ta có
C=2n+1.cosa2.1cos2na.12n+1.sin2nasina2
=cota2.tan2na