x=tan(θ){-π/2＜θ＜π/2}
x^2=(sin(θ))^2/(cos(θ))^2
1+x^2=1+(sin(θ))^2/(cos(θ))^2=1/(cos(θ))^2
(cos(θ))^2=1/(1+x^2)
(sin(θ))^2=1-(cos(θ))^2=1-1/(1+x^2)=x^2/(1+x^2)
sin(θ)=±√(x^2/(1+x^2))=±x/√((1+x^2))
となるが、このとき{-π/2＜θ＜π/2}でcos(θ)≧0なので
xつまりtan(θ)(=sin(θ)/cos(θ))とsin(θ)の符号は一致する
∴sin(θ)=x/√((1+x^2))

(-1/3)/t^3+1/t
=(-1/3)/(sin(θ))^3+1/(sin(θ))
=(-1/3)/(x/√((1+x^2)))^3+1/(x/√((1+x^2)))
=(-1/3)(1+x^2)^(3/2)/x^3+(1+x^2)^(1/2)/x