Curtate Cycloid Evolute

$\displaystyle x$	$\textstyle =$	$\displaystyle a\phi-b\sin\phi$	(1)
$\displaystyle y$	$\textstyle =$	$\displaystyle a-b\cos\phi.$	(2)

is given by

$\displaystyle x$	$\textstyle =$	$\displaystyle {a[-2b\phi+2a\phi\cos\phi-2a\sin\phi+b\sin(2\phi)]\over 2(a\cos\phi-b)}$	(3)
$\displaystyle y$	$\textstyle =$	$\displaystyle {a(a-b\cos\phi)^2\over b(a\cos\phi-b)}.$	(4)