y=(cosx)三次方,,求Y的N次求导...追50分,
问题描述：
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)
答
cos3x = 4(cosx)^3 - 3cosxy = (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 * ...