Taco \(Tan\alpha=\dfrac{5}{12}\Rightarrow1+tan^2\alpha=\dfrac{1}{cos^2\alpha}\)
\(\Leftrightarrow cos^2\alpha=\dfrac{1}{1+tan^2\alpha}=\dfrac{1}{1+\dfrac{25}{144}}=\dfrac{144}{169}\)
\(TacoN=6sin^2\alpha+7cos^2\alpha=6\left(sin^2\alpha+cos^2\alpha\right)+cos^2\alpha=6+\dfrac{144}{169}=\dfrac{1158}{169}\)
\(tana=\dfrac{5}{12}\Rightarrow tan^2a=\dfrac{25}{144}\Rightarrow\dfrac{sin^2a}{cos^2a}=\dfrac{25}{144}\)
\(\Rightarrow\dfrac{1-cos^2a}{cos^2a}=\dfrac{25}{144}\Rightarrow cos^2a=\dfrac{144}{169}\)
\(B=6\left(sin^2a+cos^2a\right)+cos^2a=6.1+\dfrac{144}{169}=...\)
