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Find thw value of sin18?

Now let's look at the solution.OK.

Let theeta = 18*. Then,

5 theeta = 90*

2 theeta + 3 theeta = 90*

2 theeta = 90* - 3 theeta

sin 2 theeta = sin(90* - 3 theeta)

sin 2 theeta = cos 3 theeta

2 sin theeta cos theeta = 4 cos3 theeta - 3 cos theeta

cos theeta(2 sin theeta - 4 cos2 theeta + 3) = 0

2 sin theeta - 4 cos2 theeta + 3 = 0

2 sin theeta - 4(1- sin2 theeta) + 3 = 0

4 sin2 theeta + 2 sin theeta - 1 = 0

sin theeta = - 2 +- root over 4+16 (divided by) 8

simplify & then u will get that the value of sin 18* comes out to be root 5 - 1(divided by) 4

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