Correct option: A.$y = A{e^{2/3(2a - x)\sqrt {x + a} }}$
a
(a) Given $\frac{{dy}}{{dx}} + \frac{{xy}}{{\sqrt {a + x} }} = 0$==>$\frac{{dy}}{y} = \frac{{ - xdx}}{{\sqrt {a + x} }}$
Integrating both sides, $\int {\frac{{dy}}{y}} = \int {\frac{{ - x}}{{\sqrt {x + a} }}dx} $
$\log y = - \int_{}^{} {\frac{{x + a - a}}{{\sqrt {x + a} }}} dx$$ = - \int_{}^{} {\sqrt {x + a} } dx + \int_{}^{} {\frac{a}{{\sqrt {x + a} }}} dx$
==> $\log y = - \frac{2}{3}{(x + a)^{3/2}} + 2a\sqrt {x + a} + \log A$
$y = A{e^{ - 2/3{{(x + a)}^{3/2}} + 2a\sqrt {x + a} }}$$ = A{e^{\left[ {(\sqrt {x + a} \left( { - \frac{2}{3}(x + a) + 2a} \right)} \right]}}$
$ = A{e^{\left[ {\sqrt {x + a} \left( {\frac{{ - 2x - 2a + 6a}}{3}} \right)} \right]}}$$ = A{e^{[ - 2/3\sqrt {x + a} (x - 2a)]}}$
or $y = A{e^{[2/3\sqrt {x + a} (2a - x)]}}$.