Which of the following quantities are rational?
sin11π12sin5π12
cosec9π10sec4π5
sin4π8+cos4π8
1+cos2π91+cos4π91+cos8π9
(a) sin11π12sin5π12=sinπ12cosπ12 =12sinπ6=14∈Q(b) cosec9π10sec4π5=−cosecπ10secπ5=1−sin18∘cos36∘=−16(5−1)(5+1)=−4∈Q(c) sin4π8+cos4π8=1−2sin2π8cos2π8=1−12sin2π4=1−14=34∈Q(d) 2cos2π92cos22π92cos24π9=8cos20∘cos40∘cos80∘2=18∈Q