Use Theorem s(x+y)= s(x)c(y)+c(x)s(y) to show

cos(4x)= http://www3.wolframalpha.com/Calcula...s=6&w=155&h=18

Then apply this formula to find the value of cos(http://www3.wolframalpha.com/Calcula...&s=31&w=9&h=18/8)

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- November 13th 2012, 08:43 PMchizmin10Theorem
Use Theorem s(x+y)= s(x)c(y)+c(x)s(y) to show

cos(4x)= http://www3.wolframalpha.com/Calcula...s=6&w=155&h=18

Then apply this formula to find the value of cos(http://www3.wolframalpha.com/Calcula...&s=31&w=9&h=18/8) - November 13th 2012, 08:58 PMMarkFLRe: Theorem

Now apply the angle-sum identity for sine as required.

Then use:

in the resulting formula. I would suggest letting to get a quadratic in . Take the appropriate root.