tan a = $\frac{2}{5}$ Tính B = $\frac{5 sin a + 2cosa}{2sina -5cosa}$ mn giúp em vs

$tan\alpha=\dfrac{2}{5}↔\dfrac{sin\alpha}{cos\alpha}=\dfrac{2}{5}$

$→sin\alpha=\dfrac{2}{5}cos\alpha$

$B=\dfrac{5sin\alpha+2cos\alpha}{2sin\alpha-5cos\alpha}$

$=\dfrac{5.\dfrac{2}{5}cos\alpha+2cos\alpha}{2.\dfrac{2}{5}cos\alpha-5cos\alpha}$

$=\dfrac{2cos\alpha+2cos\alpha}{\dfrac{4}{5}cos\alpha-5cos\alpha}$

$=\dfrac{4cos\alpha}{\dfrac{-21}{5}cos\alpha}$

$=\dfrac{-20}{21}$