>>975
y = sin(x) とする

y = sin(x) = x - (1/6)x^3 + O(x^5)
これから
y^2 = x^2 - (1/3)x^4 + O(x^6),
y^3 = x^3 + O(x^5),
y^4 = x^4 + O(x^6)

log(1+sin(x)) = log(1+y)
= y - (1/2)y^2 + (1/3)y^3 - (1/4)y^4 + O(y^5)
= {x - (1/6)x^3} - (1/2){x^2 - (1/3)x^4} + (1/3)x^3 - (1/4)x^4 + O(x^5)
= x - (1/2)x^2 + (1/6)x^3 - (1/12)x^4 + O(x^5)