



 
Hi Citra. Did you notice that all of those numbers are multiples of 18? So if I can show you how to find sin 18°, you should be able to use that in conjunction with double angle identities and sum identities to find the whole thing. [ credit for the following: http://www.andrews.edu/~calkins/math/webtexts/NUMB18.HTM#SIN18 ]
If you consider the unit circle, this means sin 18° is shown on a triangle of radius 1, with the vertical side going up to (√ 5  1) / 4. Pythagoras can tell you what the horizontal side is, which is cos 18°. Now try using double angle formulas and addition (3*18 = 2*18 + 18) formulas to expand: sin(3*18°) cos(2*18°) / cos(18°)  2 cos(2*18°)  2 sin(18°). Then you'll have the exact value of the expression you sent to us. Cheers,  


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