sin x = x-x^3/3!+x^5/5!-...(-1)^(k-1)*x^(2k-1)/(2k-1)!+Rn(x)(-∞cos x = 1-x^2/2!+x^4/4!-...(-1)^k*x^(2k)/(2k)!+...+Rn(x) (-∞Rn(x)为余项
