Given,
cosy=xcos( a+y )(1)
Differentiate both sides with respect to x.
d dx { cosy }= d dx { xcos( a+y ) } −siny dy dx =cos( a+y ) d dx ( x )+x× d dx { cos( a+y ) } −siny dy dx =cos( a+y )+x{ −sin( a+y ) } dy dx [ xsin( a+y )−siny ] dy dx =cos( a+y )
From equation (1), we get,
cosy=xcos( a+y ) x= cosy cos( a+y )
Substitute the value of x in the above equation.
[ cosy cos( a+y ) ×sin( a+y )−siny ] dy dx =cos( a+y ) [ cosy×sin( a+y )−siny×cos( a+y ) ] dy dx = cos 2 ( a+y ) sin( a+y−y ) dy dx = cos 2 ( a+y ) dy dx = cos 2 ( a+y ) sina
Hence, it is proved that dy dx = cos 2 ( a+y ) sina .