Answer
$\frac{a}{c}$
Work Step by Step
From the given Question, we can say that the Hypotenuse is Side $c$, and adjacent side of $\angle B$ is Side $a$; this is because $Cosβ=\frac{a}{c}=\frac{Adjacent side of \angle β}{Hypotenuse}$
$Sinα=\frac{Opposite side of \angle α}{Hypotenuse}$
Opposite Side of $\angle α$ = Adjacent side of $\angle β$ = a
Hypotenuse= c
Therefore, $Sinα=\frac{a}{c}$
