Ta có: $sin^{2}\alpha$ + $cos^{2}\alpha$ = 1
$\Rightarrow$ A = 2( 1 - $cos^{2}\alpha$) + 5. $cos^{2}\alpha$
= 2. ( 1 - $\frac{1}{2}$) + 5. $\frac{1}{2}$ = $\frac{7}{2}$
Ta có: $sin^{2}\alpha$ + $cos^{2}\alpha$ = 1
$\Rightarrow$ A = 2( 1 - $cos^{2}\alpha$) + 5. $cos^{2}\alpha$
= 2. ( 1 - $\frac{1}{2}$) + 5. $\frac{1}{2}$ = $\frac{7}{2}$