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y=(cosx)三次方,,求Y的N次求导...追50分,

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y=(cosx)三次方,,求Y的N次求导...追50分,
如题...
3/4 COS(X+N PI/2)+(3/4)^N COS (3X+N PI/2)
cos3x = 4(cosx)^3 - 3cosx
y = (cosx)^3 = 1/4 * (cos3x) + 3/4 * cosx
(cosx)' = -sinx = cos(x+Pi/2)
(cosx)'' = - cosx = cos(x+2*Pi/2)
(cosx)n次导 = cos(x + n * Pi/2)
y(n次导) = 3/4 * cos(x + n*Pi/2) + 1/4 * 3^n * cos(3x + n*Pi/2)