Question 1183332
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Use the general trigonometric formula


    cos(2a) = 2cos^2(a) - 1.


twice.  It gives 


    cos(4t) = 2cos^2(2t) - 1 = 2*(2cos^2(t)-1)^2 - 1 = 2*(4cos^4(t) - 4cos^2(t) +1) - 1 = 

            = 8cos^4(t) - 8cos^2(t) + 1.     <U>ANSWER</U>
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Solved and carefully explained.