Question

cos1°cos2°cos3°................cos90°cos91°.......COS180°=?

Answer 1

cos180° = 0

Cos1.cos2.cos3.cos4.........cos90........cos178.cos179.cos180

cos 90 = 0
when any thing is multilpied to 0 the result is always 0

so cos1.cos2.cos3........cos180=0

Answer 2

see
$\cos(1) cos2cos3.....cos90....cos180 \\ we \: know \: cos90 = 0 \\ so$
above whole question reduces to zero