If you want just the value then you can use scientific calculator, but if you want to find it then you have use the formula for sin 5x :), where x=18. You can see the derivation of sin 5x in terms of x at http://www.exposix.com/members/answers/H… . Now, you have just solve a cubic equation, that is simple algebra :)
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sin 18 = -0.75098 (use your scientific calculator.)
(sin18)' = 0 (no variable, it's a constant!)
(sin18)' = 0 (no variable, it's a constant!)