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$A=\dfrac{3}{\sin^2a-\sin a.\cos a-\cos^2a}=\dfrac{3(\sin^2a+\cos^2a)}{\sin^2a-\sin a.\cos a-\cos^2a}$
$A=\dfrac{3\left (1+\left ( \dfrac{\cos a}{\sin a} \right )^2 \right )}{1-\left ( \dfrac{\cos a}{\sin a} \right )-\left ( \dfrac{\cos a}{\sin a} \right )^2}=\dfrac{3\left (1+\left (\cot a \right )^2 \right
)}{1-\left ( \cot a\right )-\left ( \cot a \right )^2}$
$A=\dfrac{3\left (1+\left ( \dfrac{1}{3} \right )^2 \right
)}{1-\left ( \dfrac{1}{3} \right )-\left ( \dfrac{1}{3} \right )^2}$
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