Respuesta :

calculista
in the triangle XYB 
cos(x)=XY/XB

in the triangle AYZ
cos(z)=YZ/AZ
we know that 
cos(x)=cos(z) and XY=YZ

therefore
XY/XB=YZ/AZ-------------- > (XY/YZ)=(XB/AZ)     remember that XY=YZ
1=(XB/AZ)---------- > XB=AZ---------- > the statement was proved