Question

cos(180/11)cos (360/11)cos(540/11)cos(720/11)cos(900/11)​

Answer 1

cos (180/5) * cos (360/5) =?

cos (180/5) * cos (360/5)

= cos 36 * cos72

= cos 36 * sin 36 * cos72 / sin 36

= 1 / 2sin72 * cos72 / sin 36

= 1 / 4sin144 / sin 36

= 1 / 4sin36 / sin 36

= 1/4

1/4

hope it helps you