[tex] \displaystyle\\
m(\sphericalangle A)=90^o~~~\Longrightarrow~~~BC~~\text{este ipotenuza.}\\\\
AM = 15~cm~~~\Longrightarrow~~~BC=2\cdot AM =2\cdot 15 =30~cm\\\\
cos(B)=\frac{4}{5} \\\\
cos(B)=\frac{AB}{BC}=\frac{4}{5} \\\\
AB= \frac{BC\cdot 4}{5}=\frac{30\cdot 4}{5}=6\cdot 4 = \boxed{\bf 24~cm}
[/tex]
