>>365　別解

1/sinx ≒1/(x-x^3/6+ O(x^5) )
= 1/ ( x*(1-x^2/6 + O(x^5)/x) )
=(1/x) * 1/ (1- x^2/6 +O(x^5)/x )
=(1/x) *(1 +x^2/6 -O(x^6) ) → (1/x) *(1 +x^2/6 +O(x^6) )
1/(1-a) ≒　1+a　を使う