`tan a=1/15=>sin a=1/15cos a<=>sin^2 a=1/225cos^2 a`
`1+tan^2 a=1/[cos^2 a]=>1+(1/15)^2=1/[cos^2 a]=>cos^2 a=225/226`
$\bullet$ `sin 2a=2sin a.cos a=2. 1/15cos a.cos a=2/15 .cos^2 a=2/15 . 225/226=15/113`
$\bullet$ `cos 2a=2cos^2 a-1=2. 225/226-1=112/113`