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sin3 = sin 3*1
= 3sin1 - 4sin^3 1
but sin3 = sin (18 - 15)
= sin18 cos15 - cos18 sin15
therefore,
sin18 cos15 - cos18 sin15 = 3 sin1 - 4 sin^3 1
-> sin 18 =[ 5^(1/2) - 1] / 4
cos18= [ {10 + 2 (5)^ 1/2 } ^ 1/2] / 4
sin 15= (3^1/2 -1) / 2root2
cos 15= (3^1/2 +1)/ 2root2