[tex]A=\frac{sin2a+3}{cos^{2}a-3sin^{2}a}=\frac{2sinacosa+3}{cos^2a-3sin^2a}=\frac{2tana.cos^2a+3}{cos^2a-3tan^2a.cos^2a}=\frac{4cos^2a+3}{-11cos^2a}[/tex]
Mà [tex]\frac{1}{cos^2a}=1+tan^2a=5\Rightarrow cos^2a=\frac{1}{5}\Rightarrow A=-\frac{19}{11}[/tex]