y=cosax*cosbx=1/2[cos(a+b)x+cos(a-b)x]y(1)=1/2[(-1)(a+b)sin(a+b)x+(-1)(a-b)sin(a-b)x]y(2)=1/2[(-1)(a+b)^2cos(a+b)x+(-1)(a-b)^2cos(a-b)x]y(3)=1/2[(-1)^2(a+b)^3sin(a+b)x+(-1)^2(a-b)^3sin(a-b)x]y(4)=1/2[...