Question

Cos 9 degrees- Sin 9 degrees =?

Answer 1

Answer:

Consider, sin72=2sin36cos36

⇒sin(90−18)=4sin18cos18(1−2sin218)

⇒cos18=4sin18cos18(1−2sin218)

⇒8sin318−4sin18+1=0

⇒(2sin18−1)(4sin218+2sin18−1)=0∵ sin18=21

∴ 4sin218+2sin18−1=0

⇒sin18=4−1±5