>>21
√(1-x^2)=Σ[n=0→∞] ((n-3/2)!/(n!(-3/2)!)) x^(2n) だから
∫√(1-x^2)dx=Σ[n=0→∞] ((n-3/2)!/(n!(-3/2)!(2n+1))) x^(2n+1) + C