数学函数求导 

y=sinaxcosbx=(1/2)[sin(a+b)x+sin(a-b)x]y'=(1/2)[(a+b)cos(a+b)x+(a-b)cos(a-b)x]y"=-(1/2)[(a+b)^2sin(a+b)x+(a-b)^2sin(a-b)x]y(3)=-(1/2)[(a+b)^3cos(a+b)x+(a-b)^3cos(a-b)x]y(4)=(1/2)[(a+b)^4sin(a+b)x+(a-b)^4sin(a-b)x]y(n)导数可分四种不同表达式n=4m时,(m=1,2,3,。。。)y(n)=(1/2)[(a+b)^n*sin(a+b)x+(a-b)^n*sin(a-b)x]n=4m-1时,y(n)=-(1/2)[(a+b)^n*cos(a+b)x+(a-b)^n*cos(a-b)x]n=4m-2时,y(n)=-(1/2)[(a+b)^n*sin(a+b)x+(a-b)^n*sin(a-b)x]n=4m-3时y(n)=(1/2)[(a+b)^n*cos(a+b)x+(a-b)^n*cos(a-b)x]