Find the values of sin9. cos9
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Answer:
Step-by-step explanation:
Consider, sin72
=2sin36
cos36
⇒sin(90
−18
)=4sin18
cos18
(1−2sin
2
18
)
⇒cos18
=4sin18
cos18
(1−2sin
2
18
)
⇒8sin
3
18
−4sin18
+1=0
⇒(2sin18
−1)(4sin
2
18
+2sin18
−1)=0
∵ sin18
=
2
1
∴ 4sin
2
18
+2sin18
−1=0
⇒sin18
=
4
−1±
5
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