利用 cosx = 1 - x²/2 + x⁴/24 - ... = ∑(-1)^(2N) * x^(2N)/(2N!) N =0 →∞y=(sinx)^2 = (1 - cos2x) / 2 = 1/2 - 1/2 + (2x)²/4 - (2x)⁴/48 - ............. = x^2 - x^4/3 + 2x^6/45 -
