罗比达法则
= x(x-sinx) / 2x^4
= (x-sinx) / 2x^3
= [ x- (x - x^3/3!+ o(x^3)) ] / 2x^3
= 1/12(x-sinx) / 2x^3= [ x- (x - x^3/3! + o(x^3)) ] / 2x^3是怎么出来的啊