简单计算一下即可，答案如图所示
f(x) = x-(ax+bsinx)cosx =x-[(a+b)x-bx^3/6+o(x^3)][1-x^2/2+o(x^3)]= x-(a+b)x-bx^3/6+(a+b)x^3/2 + o(x^3)= (1-a-b)x + (3a+2b)x^3/6 + o(x^3) ~ x^31-a-b = 0, 3a+2b = 1, a = -1， b = 2
